diff --git a/client.go b/client.go
index e21eccb..4899b89 100644
--- a/client.go
+++ b/client.go
@@ -57,7 +57,7 @@ func (c *Client) Close() error { return c.w.Close() }
 // it already exists. If successful, methods on the returned File can be
 // used for I/O; the associated file descriptor has mode O_RDWR.
 func (c *Client) Create(path string) (*File, error) {
-	return c.open(path, ssh_FXF_READ|ssh_FXF_WRITE|ssh_FXF_CREAT|ssh_FXF_TRUNC)
+	return c.open(path, flags(os.O_RDWR|os.O_CREATE|os.O_TRUNC))
 }
 
 func (c *Client) sendInit() error {
@@ -228,7 +228,14 @@ func (c *Client) Lstat(p string) (os.FileInfo, error) {
 // returned file can be used for reading; the associated file descriptor
 // has mode O_RDONLY.
 func (c *Client) Open(path string) (*File, error) {
-	return c.open(path, ssh_FXF_READ)
+	return c.open(path, flags(os.O_RDONLY))
+}
+
+// OpenFile is the generalized open call; most users will use Open or
+// Create instead. It opens the named file with specified flag (O_RDONLY
+// etc.). If successful, methods on the returned File can be used for I/O.
+func (c *Client) OpenFile(path string, f int) (*File, error) {
+	return c.open(path, flags(f))
 }
 
 func (c *Client) open(path string, pflags uint32) (*File, error) {
@@ -566,3 +573,31 @@ func unmarshalStatus(id uint32, data []byte) error {
 		lang: lang,
 	}
 }
+
+// flags converts the flags passed to OpenFile into ssh flags.
+// Unsupported flags are ignored.
+func flags(f int) uint32 {
+	var out uint32
+	switch f & os.O_WRONLY {
+	case os.O_WRONLY:
+		out |= ssh_FXF_WRITE
+	case os.O_RDONLY:
+		out |= ssh_FXF_READ
+	}
+	if f&os.O_RDWR == os.O_RDWR {
+		out |= ssh_FXF_READ | ssh_FXF_WRITE
+	}
+	if f&os.O_APPEND == os.O_APPEND {
+		out |= ssh_FXF_APPEND
+	}
+	if f&os.O_CREATE == os.O_CREATE {
+		out |= ssh_FXF_CREAT
+	}
+	if f&os.O_TRUNC == os.O_TRUNC {
+		out |= ssh_FXF_TRUNC
+	}
+	if f&os.O_EXCL == os.O_EXCL {
+		out |= ssh_FXF_EXCL
+	}
+	return out
+}
diff --git a/client_integration_test.go b/client_integration_test.go
index 69f2929..cc46f72 100644
--- a/client_integration_test.go
+++ b/client_integration_test.go
@@ -233,6 +233,25 @@ func TestClientCreate(t *testing.T) {
 	defer f2.Close()
 }
 
+func TestClientAppend(t *testing.T) {
+	sftp, cmd := testClient(t, READWRITE)
+	defer cmd.Wait()
+	defer sftp.Close()
+
+	f, err := ioutil.TempFile("", "sftptest")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	defer os.Remove(f.Name())
+
+	f2, err := sftp.OpenFile(f.Name(), os.O_RDWR|os.O_APPEND)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f2.Close()
+}
+
 func TestClientCreateFailed(t *testing.T) {
 	sftp, cmd := testClient(t, READONLY)
 	defer cmd.Wait()
diff --git a/client_test.go b/client_test.go
index bbdf65f..9ade6d1 100644
--- a/client_test.go
+++ b/client_test.go
@@ -2,6 +2,7 @@ package sftp
 
 import (
 	"io"
+	"os"
 	"testing"
 
 	"github.com/kr/fs"
@@ -10,6 +11,9 @@ import (
 // assert that *Client implements fs.FileSystem
 var _ fs.FileSystem = new(Client)
 
+// assert that *File implements io.ReadWriteCloser
+var _ io.ReadWriteCloser = new(File)
+
 var ok = &StatusError{Code: ssh_FX_OK}
 var eof = &StatusError{Code: ssh_FX_EOF}
 var fail = &StatusError{Code: ssh_FX_FAILURE}
@@ -49,3 +53,23 @@ func TestOkOrErr(t *testing.T) {
 		}
 	}
 }
+
+var flagsTests = []struct {
+	flags int
+	want  uint32
+}{
+	{os.O_RDONLY, ssh_FXF_READ},
+	{os.O_WRONLY, ssh_FXF_WRITE},
+	{os.O_RDWR, ssh_FXF_READ | ssh_FXF_WRITE},
+	{os.O_RDWR | os.O_CREATE | os.O_TRUNC, ssh_FXF_READ | ssh_FXF_WRITE | ssh_FXF_CREAT | ssh_FXF_TRUNC},
+	{os.O_WRONLY | os.O_APPEND, ssh_FXF_WRITE | ssh_FXF_APPEND},
+}
+
+func TestFlags(t *testing.T) {
+	for i, tt := range flagsTests {
+		got := flags(tt.flags)
+		if got != tt.want {
+			t.Errorf("test %v: flags(%x): want: %x, got: %x", i, tt.flags, tt.want, got)
+		}
+	}
+}
diff --git a/sftp.go b/sftp.go
index ceff232..7f560fc 100644
--- a/sftp.go
+++ b/sftp.go
@@ -155,10 +155,8 @@ func (u *unexpectedPacketErr) Error() string {
 	return fmt.Sprintf("sftp: unexpected packet: want %v, got %v", fxp(u.want), fxp(u.got))
 }
 
-type unimplementedPacketErr uint8
-
-func (u unimplementedPacketErr) Error() string {
-	return fmt.Sprintf("sftp: unimplemented packet type: got %v", fxp(u))
+func unimplementedPacketErr(u uint8) error {
+	return fmt.Errorf("sftp: unimplemented packet type: got %v", fxp(u))
 }
 
 type unexpectedIdErr struct{ want, got uint32 }