diff --git a/discord/discordread.go b/discord/discordread.go
index 24be855..ee2462c 100644
--- a/discord/discordread.go
+++ b/discord/discordread.go
@@ -20,6 +20,9 @@ type Event struct {
 	MessageID   string
 	Content     string
 	Attachments []*discordgo.MessageAttachment
+	// ReplyToID is the ID of the message this one replies to, empty when the
+	// message is not a reply.
+	ReplyToID string
 }
 
 type Client struct {
@@ -49,12 +52,16 @@ func (d *Client) read(eventchannel chan<- Event) {
 		if m.ChannelID != d.ChannelID || m.Author.ID == s.State.User.ID {
 			return
 		}
-		eventchannel <- Event{
+		event := Event{
 			Kind:        MessageCreated,
 			MessageID:   m.ID,
 			Content:     m.Content,
 			Attachments: m.Attachments,
 		}
+		if m.MessageReference != nil {
+			event.ReplyToID = m.MessageReference.MessageID
+		}
+		eventchannel <- event
 	})
 
 	// Deleted messages arrive without an author, so the channel is the only
diff --git a/main.go b/main.go
index 811be58..92d5a90 100644
--- a/main.go
+++ b/main.go
@@ -114,7 +114,8 @@ func (dist *distributor) reportf(format string, args ...any) {
 	dist.d.Write(errstr)
 }
 
-// created posts a new Discord message to every output.
+// created posts a new Discord message to every output, as a reply when the
+// Discord message itself was a reply to something we already distributed.
 func (dist *distributor) created(event discord.Event) {
 	if length := utf8.RuneCountInString(event.Content); length > maxTweetLength {
 		dist.reportf("Error: message is %d characters, exceeding the %d character limit; not posted", length, maxTweetLength)
@@ -131,12 +132,23 @@ func (dist *distributor) created(event discord.Event) {
 		return
 	}
 
+	// A reply to a message we never distributed becomes a top level post.
+	var parents store.Refs
+	if event.ReplyToID != "" {
+		parents, _ = dist.store.Get(event.ReplyToID)
+	}
+
 	refs := store.Refs{}
 	for _, out := range dist.outputs {
-		ref, err := out.Write(output.Post{
+		post := output.Post{
 			Text:   event.Content,
 			Images: images,
-		})
+		}
+		if parent, ok := parents[out.GetName()]; ok && !parent.IsZero() {
+			post.ReplyTo = &parent
+		}
+
+		ref, err := out.Write(post)
 		if err != nil {
 			dist.reportf("%s Error: %s", out.GetName(), err)
 			continue
diff --git a/output/bluesky.go b/output/bluesky.go
index 176cb5c..c133479 100644
--- a/output/bluesky.go
+++ b/output/bluesky.go
@@ -67,6 +67,28 @@ func (bo *blueskyoutput) Write(post Post) (Ref, error) {
 		Langs:     []string{"ja"},
 	}
 
+	root := Ref{}
+	if post.ReplyTo != nil && post.ReplyTo.URI != "" {
+		parent := &atproto.RepoStrongRef{
+			Uri: post.ReplyTo.URI,
+			Cid: post.ReplyTo.CID,
+		}
+		// A post that is itself a reply carries the thread root; one that is
+		// not is the root of its own thread.
+		rootref := &atproto.RepoStrongRef{
+			Uri: post.ReplyTo.RootURI,
+			Cid: post.ReplyTo.RootCID,
+		}
+		if rootref.Uri == "" {
+			rootref = parent
+		}
+		feedpost.Reply = &bsky.FeedPost_ReplyRef{
+			Parent: parent,
+			Root:   rootref,
+		}
+		root = Ref{RootURI: rootref.Uri, RootCID: rootref.Cid}
+	}
+
 	if len(post.Images) > 0 {
 		embedimages := make([]*bsky.EmbedImages_Image, 0, len(post.Images))
 		for _, img := range post.Images {
@@ -96,7 +118,12 @@ func (bo *blueskyoutput) Write(post Post) (Ref, error) {
 		return Ref{}, recerr
 	}
 
-	return Ref{URI: record.Uri, CID: record.Cid}, nil
+	return Ref{
+		URI:     record.Uri,
+		CID:     record.Cid,
+		RootURI: root.RootURI,
+		RootCID: root.RootCID,
+	}, nil
 }
 
 func (bo *blueskyoutput) Delete(ref Ref) error {
diff --git a/output/output.go b/output/output.go
index 714f3ec..ec063de 100644
--- a/output/output.go
+++ b/output/output.go
@@ -7,18 +7,28 @@ type Image struct {
 }
 
 // Ref identifies a post an output has already published so it can later be
-// deleted. The fields are output specific; only the ones the publishing
-// output filled in are meaningful to it.
+// replied to or deleted. The fields are output specific; only the ones the
+// publishing output filled in are meaningful to it.
 type Ref struct {
-	ID  string `json:"id,omitempty"`  // twitter: tweet ID
-	URI string `json:"uri,omitempty"` // bluesky: at:// URI of the record
-	CID string `json:"cid,omitempty"` // bluesky: record CID
+	ID      string `json:"id,omitempty"`  // twitter: tweet ID
+	URI     string `json:"uri,omitempty"` // bluesky: at:// URI of the record
+	CID     string `json:"cid,omitempty"` // bluesky: record CID
+	RootURI string `json:"rooturi,omitempty"`
+	RootCID string `json:"rootcid,omitempty"`
+}
+
+// IsZero reports whether the ref points at nothing.
+func (r Ref) IsZero() bool {
+	return r == Ref{}
 }
 
 // Post is a single message to distribute.
 type Post struct {
 	Text   string
 	Images []Image
+	// ReplyTo is the Ref this same output returned for the post being
+	// replied to, or nil for a top level post.
+	ReplyTo *Ref
 }
 
 type OutputInterface interface {
diff --git a/output/stdout.go b/output/stdout.go
index 0a9e4c4..0bbf5d8 100644
--- a/output/stdout.go
+++ b/output/stdout.go
@@ -12,6 +12,9 @@ func StdOutput() *stdoutput {
 }
 
 func (so *stdoutput) Write(post Post) (Ref, error) {
+	if post.ReplyTo != nil {
+		fmt.Printf("[reply to: %s]\n", post.ReplyTo.ID)
+	}
 	fmt.Println(post.Text)
 	for _, img := range post.Images {
 		fmt.Printf("[image: %s (%s, %d bytes)]\n", img.Filename, img.ContentType, len(img.Data))
diff --git a/output/twitter.go b/output/twitter.go
index 4759a5a..30a3aab 100644
--- a/output/twitter.go
+++ b/output/twitter.go
@@ -63,6 +63,12 @@ func (to *twitteroutput) Write(post Post) (Ref, error) {
 			MediaIDs: mediaIDs,
 		}
 	}
+	if post.ReplyTo != nil && post.ReplyTo.ID != "" {
+		p.Reply = &types.CreateInputReply{
+			InReplyToTweetID: post.ReplyTo.ID,
+		}
+	}
+
 	out, err := managetweet.Create(context.Background(), to.client, p)
 	if err != nil {
 		return Ref{}, err