diff --git a/Makefile b/Makefile
index 27c9638..ced0582 100644
--- a/Makefile
+++ b/Makefile
@@ -143,6 +143,7 @@ UPROGS=\
 	$U/_ls\
 	$U/_mkdir\
 	$U/_pingpong\
+	$U/_primes\
 	$U/_rm\
 	$U/_sh\
 	$U/_sleep\
diff --git a/user/primes.c b/user/primes.c
new file mode 100644
index 0000000..9e8e808
--- /dev/null
+++ b/user/primes.c
@@ -0,0 +1,53 @@
+#include "kernel/types.h"
+#include "user/user.h"
+
+void
+filter(int *pin)
+{
+  int n, buf, pout[2];
+  pipe(pout);
+  close(pin[1]);
+  if (read(pin[0], &n, sizeof(int))) {
+    printf("prime %d\n", n);
+    if (fork() == 0) {
+      filter(pout);
+      exit(0);
+    } else {
+      close(pout[0]);
+      while (read(pin[0], &buf, sizeof(int))) {
+        if (buf % n != 0) {
+          write(pout[1], &buf, sizeof(int));
+        }
+      }
+      close(pout[1]);
+      wait(0);
+    }
+  }
+  exit(0);
+}
+
+int
+main(int argc, char *argv[])
+{
+  if (argc > 1) {
+    fprintf(2, "Usage: primes\n");
+    exit(1);
+  }
+
+  int p[2];
+  pipe(p);
+
+  if (fork() == 0) {
+    filter(p);
+    exit(0);
+  } else {
+    close(p[0]);
+    for (int i = 2; i <= 35; ++i) {
+      write(p[1], &i, sizeof(int));
+    }
+    close(p[1]);
+    wait(0);
+  }
+
+  exit(0);
+}