diff --git a/src/nightwatch.zig b/src/nightwatch.zig
index 1c64b6e..34aa096 100644
--- a/src/nightwatch.zig
+++ b/src/nightwatch.zig
@@ -440,6 +440,7 @@ const KQueueBackend = struct {
     shutdown_pipe: [2]std.posix.fd_t, // [0]=read [1]=write; write a byte to wake the thread
     thread: ?std.Thread,
     watches: std.StringHashMapUnmanaged(std.posix.fd_t), // owned path -> fd
+    watches_mutex: std.Thread.Mutex,
     // Per-directory snapshots of filenames, used to diff on NOTE_WRITE.
     // Key: owned dir path (same as watches key), value: set of owned filenames.
     // Accessed from both the main thread (add_watch) and the background thread (scan_dir).
@@ -477,7 +478,16 @@ const KQueueBackend = struct {
             .udata = 0,
         };
         _ = try std.posix.kevent(kq, &.{shutdown_kev}, &.{}, null);
-        return .{ .handler = handler, .kq = kq, .shutdown_pipe = pipe, .thread = null, .watches = .empty, .snapshots = .empty, .snapshots_mutex = .{} };
+        return .{
+            .handler = handler,
+            .kq = kq,
+            .shutdown_pipe = pipe,
+            .thread = null,
+            .watches = .empty,
+            .watches_mutex = .{},
+            .snapshots = .empty,
+            .snapshots_mutex = .{},
+        };
     }
 
     fn deinit(self: *@This(), allocator: std.mem.Allocator) void {
@@ -506,12 +516,21 @@ const KQueueBackend = struct {
 
     fn arm(self: *@This(), allocator: std.mem.Allocator) (error{AlreadyArmed} || std.Thread.SpawnError)!void {
         if (self.thread != null) return error.AlreadyArmed;
-        self.thread = try std.Thread.spawn(.{}, thread_fn, .{ self.kq, &self.watches, &self.snapshots, &self.snapshots_mutex, allocator, self.handler });
+        self.thread = try std.Thread.spawn(.{}, thread_fn, .{
+            self.kq,
+            &self.watches,
+            &self.watches_mutex,
+            &self.snapshots,
+            &self.snapshots_mutex,
+            allocator,
+            self.handler,
+        });
     }
 
     fn thread_fn(
         kq: std.posix.fd_t,
         watches: *const std.StringHashMapUnmanaged(std.posix.fd_t),
+        watches_mutex: *std.Thread.Mutex,
         snapshots: *std.StringHashMapUnmanaged(std.StringHashMapUnmanaged(void)),
         snapshots_mutex: *std.Thread.Mutex,
         allocator: std.mem.Allocator,
@@ -525,18 +544,20 @@ const KQueueBackend = struct {
                 if (ev.filter == EVFILT_READ) return; // shutdown pipe readable, exit
                 if (ev.filter != EVFILT_VNODE) continue;
                 // Find the directory path for this fd.
+                watches_mutex.lock();
                 var wit = watches.iterator();
-                while (wit.next()) |entry| {
-                    if (entry.value_ptr.* != @as(std.posix.fd_t, @intCast(ev.ident))) continue;
-                    const dir_path = entry.key_ptr.*;
-                    if (ev.fflags & NOTE_DELETE != 0) {
-                        handler.change(dir_path, EventType.deleted) catch return;
-                    } else if (ev.fflags & NOTE_RENAME != 0) {
-                        handler.change(dir_path, EventType.renamed) catch return;
-                    } else if (ev.fflags & NOTE_WRITE != 0) {
-                        scan_dir(dir_path, snapshots, snapshots_mutex, allocator, handler) catch {};
-                    }
-                    break;
+                const dir_path: ?[]const u8 = while (wit.next()) |entry| {
+                    if (entry.value_ptr.* == @as(std.posix.fd_t, @intCast(ev.ident)))
+                        break entry.key_ptr.*;
+                } else null;
+                watches_mutex.unlock();
+                if (dir_path == null) continue;
+                if (ev.fflags & NOTE_DELETE != 0) {
+                    handler.change(dir_path.?, EventType.deleted) catch return;
+                } else if (ev.fflags & NOTE_RENAME != 0) {
+                    handler.change(dir_path.?, EventType.renamed) catch return;
+                } else if (ev.fflags & NOTE_WRITE != 0) {
+                    scan_dir(dir_path.?, snapshots, snapshots_mutex, allocator, handler) catch {};
                 }
             }
         }
@@ -617,7 +638,10 @@ const KQueueBackend = struct {
     }
 
     fn add_watch(self: *@This(), allocator: std.mem.Allocator, path: []const u8) error{ WatchFailed, OutOfMemory }!void {
-        if (self.watches.contains(path)) return;
+        self.watches_mutex.lock();
+        const already = self.watches.contains(path);
+        self.watches_mutex.unlock();
+        if (already) return;
         const path_fd = std.posix.open(path, .{ .ACCMODE = .RDONLY }, 0) catch |e| switch (e) {
             error.AccessDenied,
             error.PermissionDenied,
@@ -669,8 +693,13 @@ const KQueueBackend = struct {
             },
         };
         const owned_path = try allocator.dupe(u8, path);
-        errdefer allocator.free(owned_path);
-        try self.watches.put(allocator, owned_path, path_fd);
+        self.watches_mutex.lock();
+        self.watches.put(allocator, owned_path, path_fd) catch |e| {
+            self.watches_mutex.unlock();
+            allocator.free(owned_path);
+            return e;
+        };
+        self.watches_mutex.unlock();
         // Take initial snapshot so first NOTE_WRITE has a baseline to diff against.
         self.take_snapshot(allocator, owned_path) catch |e| switch (e) {
             error.AccessDenied,
@@ -704,7 +733,10 @@ const KQueueBackend = struct {
     }
 
     fn remove_watch(self: *@This(), allocator: std.mem.Allocator, path: []const u8) void {
-        if (self.watches.fetchRemove(path)) |entry| {
+        self.watches_mutex.lock();
+        const watches_entry = self.watches.fetchRemove(path);
+        self.watches_mutex.unlock();
+        if (watches_entry) |entry| {
             std.posix.close(entry.value);
             allocator.free(entry.key);
         }