Recent work on shared-resource contention resolution has yielded fruitful results for local area networks and radio networks, although either the solution is suboptimal [2] or a (possibly loose) upper bound on the number of users needs to be known [5]. In this work, we present the first (two) protocols for contention resolution in radio networks that are asymptotically optimal (with high probability), work without collision detection, and do not require information about the number of contenders. In addition to the theoretical analysis, the protocols are evaluated and contrasted with the previous work by extensive simulations.