diff --git a/src/natives/io.ts b/src/natives/io.ts
index 6c3941f..ce6571c 100644
--- a/src/natives/io.ts
+++ b/src/natives/io.ts
@@ -356,10 +356,74 @@ function pathOf(t: JThread, file: JObject): string {
   return t.vm.jsString(t.vm.getField(file, "java/io/File", "path"));
 }
 
+// java.util.prefs lock files: fcntl locks, which only this VM's threads can
+// contend for here. path -> readers, or -1 while one writer holds it.
+interface PrefsLock { path: string; shared: boolean }
+function prefsLocks(t: JThread) {
+  let s = t.vm.state.get("prefsLocks") as { held: Map<string, number>; fds: Map<number, PrefsLock> } | undefined;
+  if (!s) t.vm.state.set("prefsLocks", (s = { held: new Map(), fds: new Map() }));
+  return s;
+}
+// Linux errno numbers of the codes Node reports by name
+const ERRNO_NUMBER: Record<string, number> = {
+  EPERM: 1, ENOENT: 2, EIO: 5, EBADF: 9, EAGAIN: 11, ENOMEM: 12, EACCES: 13, ENOTDIR: 20, EINVAL: 22, ENFILE: 23,
+  EMFILE: 24, ENOSPC: 28, EROFS: 30, ENAMETOOLONG: 36, ELOOP: 40, ENOTSUP: 95, EOPNOTSUPP: 95, EDQUOT: 122,
+};
+
+function intPair(t: JThread, a: number, b: number): JArray {
+  const arr = t.vm.newArray(t.vm.primClass("I"), 2);
+  (arr.a as Int32Array).set([a, b]);
+  return arr;
+}
+
 const FOS = "java/io/FileOutputStream", FIS = "java/io/FileInputStream", RAF = "java/io/RandomAccessFile";
 const UFS = "java/io/UnixFileSystem";
 
 export const natives: Natives = {
+  // the errno of chmod(2), or 0
+  "java/util/prefs/FileSystemPreferences.chmod(Ljava/lang/String;I)I": (t, [name, mode]) => {
+    const abs = resolvePath(t, t.vm.jsString(name));
+    if (!t.vm.fs.stat(abs)) return ERRNO_NUMBER.ENOENT;
+    const ok = t.vm.fs.chmod(abs, (m) => (m & ~0o7777) | (mode & 0o7777));
+    return ok === false ? ERRNO_NUMBER[t.vm.fs.chmodError(abs) ?? "EIO"] ?? ERRNO_NUMBER.EIO : 0;
+  },
+  // [fd, errno]; fd 0 when the file could not be opened or locked
+  "java/util/prefs/FileSystemPreferences.lockFile0(Ljava/lang/String;IZ)[I": (t, [name, mode, shared]) => {
+    const abs = resolvePath(t, t.vm.jsString(name));
+    let fd: number;
+    try {
+      fd = openFile(t, abs, !shared, false, false);
+    } catch {
+      return intPair(t, 0, t.vm.fs.stat(abs) ? ERRNO_NUMBER.EACCES : ERRNO_NUMBER.ENOENT);
+    }
+    // the requested mode, as HotSpot's fchmod gives it; a failure is ignored there too
+    if (!shared) t.vm.fs.chmod(abs, (m) => (m & ~0o7777) | (mode & 0o7777));
+    const { held, fds } = prefsLocks(t);
+    const now = held.get(abs) ?? 0;
+    if (shared ? now < 0 : now !== 0) {
+      closeFd(t, fd);
+      return intPair(t, 0, ERRNO_NUMBER.EAGAIN);
+    }
+    held.set(abs, shared ? now + 1 : -1);
+    fds.set(fd, { path: abs, shared: !!shared });
+    return intPair(t, fd, 0);
+  },
+  "java/util/prefs/FileSystemPreferences.unlockFile0(I)I": (t, [fd]) => {
+    const { held, fds } = prefsLocks(t);
+    const lock = fds.get(fd);
+    if (lock) {
+      fds.delete(fd);
+      const now = held.get(lock.path) ?? 0;
+      if (lock.shared && now > 1) held.set(lock.path, now - 1);
+      else held.delete(lock.path);
+    }
+    try {
+      closeFd(t, fd);
+    } catch {
+      return ERRNO_NUMBER.EIO;
+    }
+    return 0;
+  },
   // serialization: the default serialVersionUID depends on whether the class
   // itself declares a static initializer
   "java/io/ObjectStreamClass.initNative()V": () => undefined,
diff --git a/test/java/Prefs.java b/test/java/Prefs.java
new file mode 100644
index 0000000..4d456db
--- /dev/null
+++ b/test/java/Prefs.java
@@ -0,0 +1,41 @@
+import java.nio.file.*;
+import java.util.Comparator;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+import java.util.prefs.Preferences;
+
+// User preferences are stored in files under java.util.prefs.userRoot, which
+// FileSystemPreferences locks and chmods as on HotSpot.
+public class Prefs {
+  // Its log lines carry the time; HotSpot logs the same ones. Held here, since
+  // the log manager keeps loggers weakly and a collected one forgets its level.
+  static final Logger PREFS_LOG = Logger.getLogger("java.util.prefs");
+
+  public static void main(String[] args) throws Exception {
+    PREFS_LOG.setLevel(Level.OFF);
+    // a fresh root, not the user's own preferences
+    Path root = Files.createTempDirectory("prefs");
+    System.setProperty("java.util.prefs.userRoot", root.toString());
+    Preferences node = Preferences.userRoot().node("java-web-test");
+    try {
+      node.put("greeting", "hello");
+      node.putInt("count", 3);
+      node.flush();
+      node.sync();
+      System.out.println(node.get("greeting", "?") + " " + node.getInt("count", 0) + " " + String.join(",", node.keys()));
+      // what reached the backing file, read apart from the cached node
+      try (var files = Files.walk(root)) {
+        for (Path f : files.filter(f -> f.endsWith(Path.of("java-web-test", "prefs.xml"))).toList()) {
+          System.out.println(root.relativize(f));
+          for (String line : Files.readAllLines(f)) if (line.contains("<entry")) System.out.println(line.strip());
+        }
+      }
+    } finally {
+      node.removeNode();
+      Preferences.userRoot().flush();
+      try (var files = Files.walk(root)) {
+        files.sorted(Comparator.reverseOrder()).forEach(p -> p.toFile().delete());
+      }
+    }
+  }
+}
diff --git a/test/jtreg/passing.txt b/test/jtreg/passing.txt
index d35c862..3b6155c 100644
--- a/test/jtreg/passing.txt
+++ b/test/jtreg/passing.txt
@@ -3246,6 +3246,14 @@ java/util/logging/TestLoggingWithMainAppContext.java
 java/util/logging/TestMainAppContext.java
 java/util/logging/TestUILoggerContext.java
 java/util/logging/XMLFormatterDate.java
+java/util/prefs/AddNodeChangeListener.java
+java/util/prefs/CodePointZeroPrefsTest.java
+java/util/prefs/CommentsInXml.java
+java/util/prefs/ConflictInFlush.java
+java/util/prefs/ExportNode.java
+java/util/prefs/ExportSubtree.java
+java/util/prefs/RemoveNullKeyCheck.java
+java/util/prefs/RemoveUnregedListener.java
 java/util/prefs/SerializeExceptions.java
 java/util/regex/ImmutableMatchResultTest.java
 java/util/regex/NamedGroupsTests.java