diff --git a/tools/rbcrun/host.go b/tools/rbcrun/host.go
index 1d68d43..7f5e332 100644
--- a/tools/rbcrun/host.go
+++ b/tools/rbcrun/host.go
@@ -31,11 +31,11 @@
 type ExecutionMode int
 const (
 	ExecutionModeRbc ExecutionMode = iota
-	ExecutionModeMake ExecutionMode = iota
+	ExecutionModeScl ExecutionMode = iota
 )
 
 const allowExternalEntrypointKey = "allowExternalEntrypoint"
-const callerDirKey = "callerDir"
+const callingFileKey = "callingFile"
 const executionModeKey = "executionMode"
 const shellKey = "shell"
 
@@ -58,7 +58,7 @@
 	"rblf_wildcard": starlark.NewBuiltin("rblf_wildcard", wildcard),
 }
 
-var makeBuiltins starlark.StringDict = starlark.StringDict{
+var sclBuiltins starlark.StringDict = starlark.StringDict{
 	"struct":   starlark.NewBuiltin("struct", starlarkstruct.Make),
 	"json": starlarkjson.Module,
 }
@@ -128,7 +128,8 @@
 			module = module[:pipePos]
 		}
 	}
-	modulePath, err := cleanModuleName(module, thread.Local(callerDirKey).(string), allowExternalEntrypoint)
+	callingFile := thread.Local(callingFileKey).(string)
+	modulePath, err := cleanModuleName(module, filepath.Dir(callingFile), allowExternalEntrypoint)
 	if err != nil {
 		return nil, err
 	}
@@ -150,6 +151,13 @@
 
 		// Load or return default
 		if mustLoad {
+			if strings.HasSuffix(callingFile, ".scl") && !strings.HasSuffix(modulePath, ".scl") {
+				return nil, fmt.Errorf(".scl files can only load other .scl files: %q loads %q", callingFile, modulePath)
+			}
+			// Switch into scl mode from here on
+			if strings.HasSuffix(modulePath, ".scl") {
+				mode = ExecutionModeScl
+			}
 			childThread := &starlark.Thread{Name: "exec " + module, Load: thread.Load}
 			// Cheating for the sake of testing:
 			// propagate starlarktest's Reporter key, otherwise testing
@@ -161,14 +169,14 @@
 
 			// Only the entrypoint starlark file allows external loads.
 			childThread.SetLocal(allowExternalEntrypointKey, false)
-			childThread.SetLocal(callerDirKey, filepath.Dir(modulePath))
+			childThread.SetLocal(callingFileKey, modulePath)
 			childThread.SetLocal(executionModeKey, mode)
 			childThread.SetLocal(shellKey, thread.Local(shellKey))
 			if mode == ExecutionModeRbc {
 				globals, err := starlark.ExecFile(childThread, modulePath, nil, rbcBuiltins)
 				e = &modentry{globals, err}
-			} else if mode == ExecutionModeMake {
-				globals, err := starlark.ExecFile(childThread, modulePath, nil, makeBuiltins)
+			} else if mode == ExecutionModeScl {
+				globals, err := starlark.ExecFile(childThread, modulePath, nil, sclBuiltins)
 				e = &modentry{globals, err}
 			} else {
 				return nil, fmt.Errorf("unknown executionMode %d", mode)
@@ -338,7 +346,7 @@
 			if mode == ExecutionModeRbc {
 				// In rbc mode, rblf_log is used to print to stderr
 				fmt.Println(msg)
-			} else if mode == ExecutionModeMake {
+			} else if mode == ExecutionModeScl {
 				fmt.Fprintln(os.Stderr, msg)
 			}
 		},
@@ -365,13 +373,13 @@
 
 	var results starlark.StringDict
 	mainThread.SetLocal(allowExternalEntrypointKey, allowExternalEntrypoint)
-	mainThread.SetLocal(callerDirKey, filepath.Dir(filename))
+	mainThread.SetLocal(callingFileKey, filename)
 	mainThread.SetLocal(executionModeKey, mode)
 	mainThread.SetLocal(shellKey, shellPath)
 	if mode == ExecutionModeRbc {
 		results, err = starlark.ExecFile(mainThread, filename, src, rbcBuiltins)
-	} else if mode == ExecutionModeMake {
-		results, err = starlark.ExecFile(mainThread, filename, src, makeBuiltins)
+	} else if mode == ExecutionModeScl {
+		results, err = starlark.ExecFile(mainThread, filename, src, sclBuiltins)
 	} else {
 		return results, nil, fmt.Errorf("unknown executionMode %d", mode)
 	}
diff --git a/tools/rbcrun/host_test.go b/tools/rbcrun/host_test.go
index 10ce55e..468a620 100644
--- a/tools/rbcrun/host_test.go
+++ b/tools/rbcrun/host_test.go
@@ -19,6 +19,7 @@
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 
 	"go.starlark.net/resolve"
@@ -126,7 +127,7 @@
 		t.Fatal(err)
 	}
 	thread.SetLocal(allowExternalEntrypointKey, false)
-	thread.SetLocal(callerDirKey, dir)
+	thread.SetLocal(callingFileKey, "testdata/load.star")
 	thread.SetLocal(executionModeKey, ExecutionModeRbc)
 	if _, err := starlark.ExecFile(thread, "testdata/load.star", nil, rbcBuiltins); err != nil {
 		if err, ok := err.(*starlark.EvalError); ok {
@@ -136,6 +137,55 @@
 	}
 }
 
+func TestBzlLoadsScl(t *testing.T) {
+	moduleCache = make(map[string]*modentry)
+	dir := dataDir()
+	if err := os.Chdir(filepath.Dir(dir)); err != nil {
+		t.Fatal(err)
+	}
+	vars, _, err := Run("testdata/bzl_loads_scl.bzl", nil, ExecutionModeScl, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if val, ok := vars["foo"]; !ok {
+		t.Fatalf("Failed to load foo variable")
+	} else if val.(starlark.String) != "bar" {
+		t.Fatalf("Expected \"bar\", got %q", val)
+	}
+}
+
+func TestNonEntrypointBzlLoadsScl(t *testing.T) {
+	moduleCache = make(map[string]*modentry)
+	dir := dataDir()
+	if err := os.Chdir(filepath.Dir(dir)); err != nil {
+		t.Fatal(err)
+	}
+	vars, _, err := Run("testdata/bzl_loads_scl_2.bzl", nil, ExecutionModeScl, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if val, ok := vars["foo"]; !ok {
+		t.Fatalf("Failed to load foo variable")
+	} else if val.(starlark.String) != "bar" {
+		t.Fatalf("Expected \"bar\", got %q", val)
+	}
+}
+
+func TestSclLoadsBzl(t *testing.T) {
+	moduleCache = make(map[string]*modentry)
+	dir := dataDir()
+	if err := os.Chdir(filepath.Dir(dir)); err != nil {
+		t.Fatal(err)
+	}
+	_, _, err := Run("testdata/scl_incorrectly_loads_bzl.scl", nil, ExecutionModeScl, false)
+	if err == nil {
+		t.Fatal("Expected failure")
+	}
+	if !strings.Contains(err.Error(), ".scl files can only load other .scl files") {
+		t.Fatalf("Expected error to contain \".scl files can only load other .scl files\": %q", err.Error())
+	}
+}
+
 func TestShell(t *testing.T) {
 	exerciseStarlarkTestFile(t, "testdata/shell.star")
 }
diff --git a/tools/rbcrun/rbcrun/rbcrun.go b/tools/rbcrun/rbcrun/rbcrun.go
index a15b867..8c372c7 100644
--- a/tools/rbcrun/rbcrun/rbcrun.go
+++ b/tools/rbcrun/rbcrun/rbcrun.go
@@ -55,13 +55,13 @@
 	case "rbc":
 		return rbcrun.ExecutionModeRbc
 	case "make":
-		return rbcrun.ExecutionModeMake
+		return rbcrun.ExecutionModeScl
 	case "":
 		quit("-mode flag is required.")
 	default:
 		quit("Unknown -mode value %q, expected 1 of \"rbc\", \"make\"", *modeFlag)
 	}
-	return rbcrun.ExecutionModeMake
+	return rbcrun.ExecutionModeScl
 }
 
 var makeStringReplacer = strings.NewReplacer("#", "\\#", "$", "$$")
@@ -175,7 +175,7 @@
 			quit("%s\n", err)
 		}
 	}
-	if mode == rbcrun.ExecutionModeMake {
+	if mode == rbcrun.ExecutionModeScl {
 		if err := printVarsInMakeFormat(variables); err != nil {
 			quit("%s\n", err)
 		}
diff --git a/tools/rbcrun/testdata/bzl_loads_scl.bzl b/tools/rbcrun/testdata/bzl_loads_scl.bzl
new file mode 100644
index 0000000..e8deca3
--- /dev/null
+++ b/tools/rbcrun/testdata/bzl_loads_scl.bzl
@@ -0,0 +1,3 @@
+load(":test_scl.scl", _foo = "foo")
+
+foo = _foo
diff --git a/tools/rbcrun/testdata/bzl_loads_scl_2.bzl b/tools/rbcrun/testdata/bzl_loads_scl_2.bzl
new file mode 100644
index 0000000..9a680ed
--- /dev/null
+++ b/tools/rbcrun/testdata/bzl_loads_scl_2.bzl
@@ -0,0 +1,3 @@
+load(":bzl_loads_scl.bzl", _foo = "foo")
+
+foo = _foo
diff --git a/tools/rbcrun/testdata/scl_incorrectly_loads_bzl.scl b/tools/rbcrun/testdata/scl_incorrectly_loads_bzl.scl
new file mode 100644
index 0000000..9a680ed
--- /dev/null
+++ b/tools/rbcrun/testdata/scl_incorrectly_loads_bzl.scl
@@ -0,0 +1,3 @@
+load(":bzl_loads_scl.bzl", _foo = "foo")
+
+foo = _foo
diff --git a/tools/rbcrun/testdata/test_scl.scl b/tools/rbcrun/testdata/test_scl.scl
new file mode 100644
index 0000000..6360ccb
--- /dev/null
+++ b/tools/rbcrun/testdata/test_scl.scl
@@ -0,0 +1,2 @@
+
+foo = "bar"