diff --git a/tests/pkill.test b/tests/pkill.test
index 7a20ae5..0bea32a 100755
--- a/tests/pkill.test
+++ b/tests/pkill.test
@@ -63,6 +63,7 @@
 
 yes >/dev/null &
 proc1=$!
+sleep 1
 yes >/dev/null &
 proc2=$!
 sleep 1
diff --git a/toys/posix/ps.c b/toys/posix/ps.c
index e5adc19..4f8d75a 100644
--- a/toys/posix/ps.c
+++ b/toys/posix/ps.c
@@ -980,6 +980,8 @@
   if (isdigit(*str)) {
     ll[pl->len] = xstrtol(str, &end, 10);
     if (end==(len+str)) num++;
+    // For pkill, -s 0 represents pkill's session id.
+    if (pl==&TT.ss && ll[pl->len]==0) ll[pl->len] = getsid(0);
   }
 
   if (pl==&TT.pp || pl==&TT.ss) {