Third-generation mobile systems provide access to a wide range of services and enable mobile users to communicate, regardless of their geographical location and their roaming characteristics. Due to the growing number of mobile users and global connectivity, one of the most critical issues regarding the design of universal mobile telecommunications service (UMTS) networks pertains to the assignment of Node Bs to radio network controllers (RNCs), which is an NP-hard problem. Hence, for real-sized mobile networks, this problem cannot be practically solved by using exact methods. This paper proposes a hybrid genetic algorithm (HA) with migration to solve the problem of assigning Bs to RNCs as a design step of large-scale UMTS mobile networks. Computational results obtained from extensive tests confirm the effectiveness of the HA to provide superior solutions compared to other heuristic methods that are well documented in the literature. Such an algorithm is particularly suitable to design large-scale cellular mobile networks with Node Bs whose quantity varies between 100 and 500 and whose the number of RNCs ranges between five and ten.