diff --git a/cmd/merge_zips/merge_zips.go b/cmd/merge_zips/merge_zips.go
index 1aa6f6f..2c57180 100644
--- a/cmd/merge_zips/merge_zips.go
+++ b/cmd/merge_zips/merge_zips.go
@@ -342,7 +342,8 @@
 func (oz *OutputZip) getUninitializedPythonPackages(inputZips []InputZip) ([]string, error) {
 	// the runfiles packages needs to be populated with "__init__.py".
 	// the runfiles dirs have been treated as packages.
-	allPackages := make(map[string]bool)
+	var allPackages []string // Using a slice to preserve input order.
+	seenPkgs := make(map[string]bool)
 	initedPackages := make(map[string]bool)
 	getPackage := func(path string) string {
 		ret := filepath.Dir(path)
@@ -369,16 +370,17 @@
 				initedPackages[pyPkg] = true
 			}
 			for pyPkg != "" {
-				if _, found := allPackages[pyPkg]; found {
+				if _, found := seenPkgs[pyPkg]; found {
 					break
 				}
-				allPackages[pyPkg] = true
+				seenPkgs[pyPkg] = true
+				allPackages = append(allPackages, pyPkg)
 				pyPkg = getPackage(pyPkg)
 			}
 		}
 	}
 	noInitPackages := make([]string, 0)
-	for pyPkg := range allPackages {
+	for _, pyPkg := range allPackages {
 		if _, found := initedPackages[pyPkg]; !found {
 			noInitPackages = append(noInitPackages, pyPkg)
 		}
diff --git a/cmd/merge_zips/merge_zips_test.go b/cmd/merge_zips/merge_zips_test.go
index 64b08d0..17228c4 100644
--- a/cmd/merge_zips/merge_zips_test.go
+++ b/cmd/merge_zips/merge_zips_test.go
@@ -103,6 +103,7 @@
 		stripFiles       []string
 		stripDirs        []string
 		jar              bool
+		par              bool
 		sort             bool
 		ignoreDuplicates bool
 		stripDirEntries  bool
@@ -265,6 +266,34 @@
 			},
 			out: []testZipEntry{withoutTimestamp, a},
 		},
+		{
+			name: "emulate par",
+			in: [][]testZipEntry{
+				{
+					testZipEntry{name: "3/main.py"},
+					testZipEntry{name: "c/main.py"},
+					testZipEntry{name: "a/main.py"},
+					testZipEntry{name: "2/main.py"},
+					testZipEntry{name: "b/main.py"},
+					testZipEntry{name: "1/main.py"},
+				},
+			},
+			out: []testZipEntry{
+				testZipEntry{name: "3/__init__.py", mode: 0700, timestamp: jar.DefaultTime},
+				testZipEntry{name: "c/__init__.py", mode: 0700, timestamp: jar.DefaultTime},
+				testZipEntry{name: "a/__init__.py", mode: 0700, timestamp: jar.DefaultTime},
+				testZipEntry{name: "2/__init__.py", mode: 0700, timestamp: jar.DefaultTime},
+				testZipEntry{name: "b/__init__.py", mode: 0700, timestamp: jar.DefaultTime},
+				testZipEntry{name: "1/__init__.py", mode: 0700, timestamp: jar.DefaultTime},
+				testZipEntry{name: "3/main.py", timestamp: jar.DefaultTime},
+				testZipEntry{name: "c/main.py", timestamp: jar.DefaultTime},
+				testZipEntry{name: "a/main.py", timestamp: jar.DefaultTime},
+				testZipEntry{name: "2/main.py", timestamp: jar.DefaultTime},
+				testZipEntry{name: "b/main.py", timestamp: jar.DefaultTime},
+				testZipEntry{name: "1/main.py", timestamp: jar.DefaultTime},
+			},
+			par: true,
+		},
 	}
 
 	for _, test := range testCases {
@@ -280,7 +309,7 @@
 			writer := zip.NewWriter(out)
 
 			err := mergeZips(inputZips, writer, "", "",
-				test.sort, test.jar, false, test.stripDirEntries, test.ignoreDuplicates,
+				test.sort, test.jar, test.par, test.stripDirEntries, test.ignoreDuplicates,
 				test.stripFiles, test.stripDirs, test.zipsToNotStrip)
 
 			closeErr := writer.Close()