diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9d2099d..07452c9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,14 @@
 # Changelog
 
+## 0.1.1 — 2026-04-06
+
+### Fixed
+
+- Prevent multiple mux instances from running simultaneously via file-based
+  exclusive lock (`fd-lock`); stale sockets from crashed instances are cleaned
+  up automatically
+- Clearer error message when TCP port is already in use
+
 ## 0.1.0 — 2026-04-06
 
 Initial release.
diff --git a/Cargo.lock b/Cargo.lock
index 39504ea..ec5d4d5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -171,11 +171,12 @@ dependencies = [
 
 [[package]]
 name = "donglora-mux"
-version = "0.1.0"
+version = "0.1.1"
 dependencies = [
  "anyhow",
  "clap",
  "donglora-client",
+ "fd-lock",
  "tokio",
  "tokio-serial",
  "tokio-util",
@@ -193,6 +194,17 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "fd-lock"
+version = "4.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0ce92ff622d6dadf7349484f42c93271a0d49b7cc4d466a936405bacbe10aa78"
+dependencies = [
+ "cfg-if",
+ "rustix",
+ "windows-sys 0.59.0",
+]
+
 [[package]]
 name = "futures"
 version = "0.3.32"
@@ -335,6 +347,12 @@ dependencies = [
  "pkg-config",
 ]
 
+[[package]]
+name = "linux-raw-sys"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "32a66949e030da00e8c7d4434b251670a91556f4144941d37452769c25d58a53"
+
 [[package]]
 name = "lock_api"
 version = "0.4.14"
@@ -522,6 +540,19 @@ version = "0.8.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc897dd8d9e8bd1ed8cdad82b5966c3e0ecae09fb1907d58efaa013543185d0a"
 
+[[package]]
+name = "rustix"
+version = "1.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b6fe4565b9518b83ef4f91bb47ce29620ca828bd32cb7e408f0062e9930ba190"
+dependencies = [
+ "bitflags 2.11.0",
+ "errno",
+ "libc",
+ "linux-raw-sys",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "scopeguard"
 version = "1.2.0"
@@ -826,6 +857,15 @@ dependencies = [
  "windows-targets",
 ]
 
+[[package]]
+name = "windows-sys"
+version = "0.59.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e38bc4d79ed67fd075bcc251a1c39b32a1776bbe92e5bef1f0bf1f8c531853b"
+dependencies = [
+ "windows-targets",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.61.2"
diff --git a/Cargo.toml b/Cargo.toml
index ebd6742..2591586 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "donglora-mux"
-version = "0.1.0"
+version = "0.1.1"
 edition = "2024"
 authors = ["Stephen Waits <steve@waits.net>"]
 license = "MIT"
@@ -18,6 +18,7 @@ tokio-serial = "5"
 tokio-util = "0.7"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+fd-lock = "4"
 
 [lints.clippy]
 unwrap_used = "deny"
diff --git a/README.md b/README.md
index 5f2b6d5..013aade 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,7 @@ dongle simultaneously.
 - RxPacket frames broadcast to all connected clients
 - StartRx/StopRx reference-counted across clients
 - SetConfig locked once set (single client can change freely)
+- Single-instance enforcement via file lock (stale sockets auto-cleaned)
 - Automatic dongle reconnect on hot-plug
 - No panics — enforced by clippy deny lints
 
diff --git a/src/daemon.rs b/src/daemon.rs
index 8b20912..92001e6 100644
--- a/src/daemon.rs
+++ b/src/daemon.rs
@@ -91,7 +91,24 @@ impl MuxDaemon {
         if let Some(parent) = sock_path.parent() {
             tokio::fs::create_dir_all(parent).await.ok();
         }
+        // Acquire an exclusive lock to prevent multiple mux instances.
+        // The lock is held for the process lifetime and auto-released on exit (even SIGKILL).
+        let lock_path = format!("{}.lock", self.socket_path);
+        let lock_file = std::fs::OpenOptions::new()
+            .create(true)
+            .write(true)
+            .truncate(false)
+            .open(&lock_path)
+            .map_err(|e| anyhow::anyhow!("failed to open lock file {lock_path}: {e}"))?;
+        let mut lock = fd_lock::RwLock::new(lock_file);
+        let _lock_guard = match lock.try_write() {
+            Ok(guard) => guard,
+            Err(_) => anyhow::bail!("another donglora-mux is already running (lock held on {lock_path})"),
+        };
+
         if sock_path.exists() {
+            // Lock acquired but socket file exists — stale from a crashed instance.
+            info!("removing stale socket {}", self.socket_path);
             tokio::fs::remove_file(sock_path).await.ok();
         }
 
@@ -103,9 +120,13 @@ impl MuxDaemon {
         // Start TCP server (optional)
         let tcp_listener = if let Some((ref host, port)) = self.tcp_addr {
             let addr = format!("{host}:{port}");
-            let listener = TcpListener::bind(&addr)
-                .await
-                .map_err(|e| anyhow::anyhow!("failed to bind TCP {addr}: {e}"))?;
+            let listener = TcpListener::bind(&addr).await.map_err(|e| {
+                if e.kind() == std::io::ErrorKind::AddrInUse {
+                    anyhow::anyhow!("another donglora-mux is already listening on TCP {addr}")
+                } else {
+                    anyhow::anyhow!("failed to bind TCP {addr}: {e}")
+                }
+            })?;
             info!("TCP listening on {addr}");
             Some(listener)
         } else {