diff --git a/file_system.go b/file_system.go
index 7bcbabb..0cdeea9 100644
--- a/file_system.go
+++ b/file_system.go
@@ -21,6 +21,12 @@ import (
 //
 // Must be safe for concurrent access via all methods.
 type FileSystem interface {
+	// This method is called once when mounting the file system. It must succeed
+	// in order for the mount to succeed.
+	Init(
+		ctx context.Context,
+		req *InitRequest) (*InitResponse, error)
+
 	// Look up a child by name within a parent directory. The kernel calls this
 	// when resolving user paths to dentry structs, which are then cached.
 	LookUpInode(
@@ -129,6 +135,15 @@ type DirOffset uint64
 // Requests and responses
 ////////////////////////////////////////////////////////////////////////
 
+type InitRequest struct {
+	// User and group IDs for the process that is mounting the file system.
+	Uid uint32
+	Gid uint32
+}
+
+type InitResponse struct {
+}
+
 type LookUpInodeRequest struct {
 	// The ID of the directory inode to which the child belongs.
 	Parent InodeID
diff --git a/fuseutil/not_implemented_file_system.go b/fuseutil/not_implemented_file_system.go
index 1fb12f4..5d549ed 100644
--- a/fuseutil/not_implemented_file_system.go
+++ b/fuseutil/not_implemented_file_system.go
@@ -15,6 +15,12 @@ type NotImplementedFileSystem struct {
 
 var _ fuse.FileSystem = &NotImplementedFileSystem{}
 
+func (fs *NotImplementedFileSystem) Init(
+	ctx context.Context,
+	req *fuse.InitRequest) (*fuse.InitResponse, error) {
+	return nil, fuse.ENOSYS
+}
+
 func (fs *NotImplementedFileSystem) LookUpInode(
 	ctx context.Context,
 	req *fuse.LookUpInodeRequest) (*fuse.LookUpInodeResponse, error) {
diff --git a/samples/hello_fs.go b/samples/hello_fs.go
index 28a7724..1354801 100644
--- a/samples/hello_fs.go
+++ b/samples/hello_fs.go
@@ -50,7 +50,7 @@ var gInodeInfo = map[fuse.InodeID]inodeInfo{
 		attributes: fuse.InodeAttributes{
 			// TODO(jacobsa): Why do we get premission denied errors when this is
 			// 0500?
-			Mode: 0555 | os.ModeDir,
+			Mode: 0500 | os.ModeDir,
 		},
 		dir: true,
 		children: []fuseutil.Dirent{
diff --git a/server.go b/server.go
index 0415643..6ba8932 100644
--- a/server.go
+++ b/server.go
@@ -71,11 +71,23 @@ func (s *server) handleFuseRequest(fuseReq bazilfuse.Request) {
 	// Attempt to handle it.
 	switch typed := fuseReq.(type) {
 	case *bazilfuse.InitRequest:
-		// Responding to this is required to make mounting work, at least on OS X.
-		// We don't currently expose the capability for the file system to
-		// intercept this.
+		// Convert the request.
+		req := &InitRequest{
+			Uid: typed.Header.Uid,
+			Gid: typed.Header.Gid,
+		}
+
+		// Call the file system.
+		_, err := s.fs.Init(ctx, req)
+		if err != nil {
+			s.logger.Print("Responding:", err)
+			typed.RespondError(err)
+			return
+		}
+
+		// Convert the response.
 		fuseResp := &bazilfuse.InitResponse{}
-		s.logger.Println("Responding:", fuseResp)
+		s.logger.Print("Responding:", fuseResp)
 		typed.Respond(fuseResp)
 
 	case *bazilfuse.StatfsRequest: