diff --git a/packages/endo/src/search.js b/packages/endo/src/search.js
new file mode 100644
index 0000000000..a32f33669a
--- /dev/null
+++ b/packages/endo/src/search.js
@@ -0,0 +1,38 @@
+import { relativize } from "./node-module-specifier.js";
+import { relative } from "./url.js";
+
+// q, as in quote, for enquoting strings in error messages.
+const q = JSON.stringify;
+
+const decoder = new TextDecoder();
+
+// Searches for the first ancestor directory of a module file that contains a
+// package.json.
+// Probes by attempting to read the file, not stat.
+// To avoid duplicate work later, returns the text of the package.json for
+// inevitable later use.
+export const search = async (read, modulePath) => {
+  let directory = new URL("./", modulePath).toString();
+  for (;;) {
+    const packageDescriptorPath = new URL("package.json", directory).toString();
+    // eslint-disable-next-line no-await-in-loop
+    const packageDescriptorBytes = await read(packageDescriptorPath).catch(
+      () => undefined
+    );
+    if (packageDescriptorBytes !== undefined) {
+      const packageDescriptorText = decoder.decode(packageDescriptorBytes);
+      return {
+        packagePath: directory,
+        packageDescriptorText,
+        moduleSpecifier: relativize(relative(directory, modulePath))
+      };
+    }
+    const parentDirectory = new URL("../", directory).toString();
+    if (parentDirectory === directory) {
+      throw new Error(
+        `Cannot find package.json along path to module ${q(modulePath)}`
+      );
+    }
+    directory = parentDirectory;
+  }
+};