I have two of their bags, the Kremlin (for stuff like hardware store runs), and the Mini-Metro bag. Actually, it looks like they no longer make the kremlin, but I would assume it's the same size as the Metropolis. When you open up the flap, there is a zip pouch, and behind that there is an open pouch. You could put a u-lock in the open pouch and it would probably fit well, depending upon the size of the bag and the lock (I use a chain, so I wouldn't know). They're great, durable bags.