Intermediate in many respects between Wilson's and Leach's storm-petrels. Blackish-brown overall with pale wing bars and a clear, curved white band across rump; white on rump more extensive than on Leach's but less than on Wilson's (where white extends fully onto undertail coverts). Tail slightly forked, but this feature is difficult to discern at sea; legs do not extend beyond tail in flight. Wings thinner and more angular than Wilson's, but thicker and less angular than Leach's. Larger than Wilson's. Difficult to identify reliably at sea.