diff --git a/bp2build/testing.go b/bp2build/testing.go
index 997df64..7333c0b 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -21,6 +21,7 @@
 
 import (
 	"fmt"
+	"path/filepath"
 	"sort"
 	"strings"
 	"testing"
@@ -82,7 +83,16 @@
 	// ExpectedBazelTargets compares the BazelTargets generated in `Dir` (if not empty).
 	// Otherwise, it checks the BazelTargets generated by `Blueprint` in the root directory.
 	ExpectedBazelTargets []string
-	Filesystem           map[string]string
+	// AlreadyExistingBuildContents, if non-empty, simulates an already-present source BUILD file
+	// in the directory under test. The BUILD file has the given contents. This BUILD file
+	// will also be treated as "BUILD file to keep" by the simulated bp2build environment.
+	AlreadyExistingBuildContents string
+	// StubbedBuildDefinitions, if non-empty, adds stub definitions to an already-present source
+	// BUILD file in the directory under test, for each target name given. These stub definitions
+	// are added to the BUILD file given in AlreadyExistingBuildContents, if it is set.
+	StubbedBuildDefinitions []string
+
+	Filesystem map[string]string
 	// Dir sets the directory which will be compared against the targets in ExpectedBazelTargets.
 	// This should used in conjunction with the Filesystem property to check for targets
 	// generated from a directory that is not the root.
@@ -110,11 +120,31 @@
 
 func runBp2BuildTestCaseWithSetup(t *testing.T, extraPreparer android.FixturePreparer, tc Bp2buildTestCase) {
 	t.Helper()
-	dir := "."
+	checkDir := "."
+	if tc.Dir != "" {
+		checkDir = tc.Dir
+	}
+	keepExistingBuildDirs := tc.KeepBuildFileForDirs
+	buildFilesToParse := []string{}
 	filesystem := make(map[string][]byte)
 	for f, content := range tc.Filesystem {
 		filesystem[f] = []byte(content)
 	}
+	alreadyExistingBuildContents := tc.AlreadyExistingBuildContents
+	if len(tc.StubbedBuildDefinitions) > 0 {
+		for _, targetName := range tc.StubbedBuildDefinitions {
+			alreadyExistingBuildContents += MakeBazelTarget(
+				"bp2build_test_stub",
+				targetName,
+				AttrNameToString{})
+		}
+	}
+	if len(alreadyExistingBuildContents) > 0 {
+		buildFilePath := filepath.Join(checkDir, "BUILD")
+		filesystem[buildFilePath] = []byte(alreadyExistingBuildContents)
+		keepExistingBuildDirs = append(keepExistingBuildDirs, checkDir)
+		buildFilesToParse = append(buildFilesToParse, buildFilePath)
+	}
 
 	preparers := []android.FixturePreparer{
 		extraPreparer,
@@ -123,7 +153,7 @@
 		android.FixtureRegisterWithContext(func(ctx android.RegistrationContext) {
 			ctx.RegisterModuleType(tc.ModuleTypeUnderTest, tc.ModuleTypeUnderTestFactory)
 		}),
-		android.FixtureModifyContext(func(ctx *android.TestContext) {
+		android.FixtureModifyContextWithMockFs(func(ctx *android.TestContext) {
 			// A default configuration for tests to not have to specify bp2build_available on top level
 			// targets.
 			bp2buildConfig := android.NewBp2BuildAllowlist().SetDefaultConfig(
@@ -131,7 +161,7 @@
 					android.Bp2BuildTopLevel: allowlists.Bp2BuildDefaultTrueRecursively,
 				},
 			)
-			for _, f := range tc.KeepBuildFileForDirs {
+			for _, f := range keepExistingBuildDirs {
 				bp2buildConfig.SetKeepExistingBuildFile(map[string]bool{
 					f: /*recursive=*/ false,
 				})
@@ -141,6 +171,10 @@
 			// from cloning modules to their original state after mutators run. This
 			// would lose some data intentionally set by these mutators.
 			ctx.SkipCloneModulesAfterMutators = true
+			err := ctx.RegisterExistingBazelTargets(".", buildFilesToParse)
+			if err != nil {
+				t.Errorf("error parsing build files in test setup: %s", err)
+			}
 		}),
 		android.FixtureModifyEnv(func(env map[string]string) {
 			if tc.UnconvertedDepsMode == errorModulesUnconvertedDeps {
@@ -159,10 +193,6 @@
 		return
 	}
 
-	checkDir := dir
-	if tc.Dir != "" {
-		checkDir = tc.Dir
-	}
 	expectedTargets := map[string][]string{
 		checkDir: tc.ExpectedBazelTargets,
 	}