In this paper, we consider the localization problem for a system of mobile robots using inexpensive range sensors. Among many issues for multi--robot systems, two problems are identified and formally defined. The first problem is sensing ranges from all robots as quickly as possible while avoiding sensor cross--talk, and the second problem is to localize a multi-robot system using a minimal number of range sensings. We show that both these problems are NP-Complete, and we propose an approximate method for the multi-robot localization problem that takes advantage of the robots' pose uncertainty information. Simulation results show the effectiveness of our method for localizing multiple robots.