The purpose of this study is to optimize the topology and shape of prestressed concrete bridge girders. An optimum design approach that uses a genetic algorithm (GA) for this purpose is presented. The cost of girders is the optimum design criterion. The design variables are the cross-sectional dimensions of the prefabricated prestressed beams, the cross-sectional area of the prestressing steel and the number of beams in the bridge cross-section. Stress, displacement and geometrical constraints are considered in the optimum design. AASHTO Standard Specifications for Highway Bridges are taken into account when calculating the loads and designing the prestressed beams. A computer program is coded in Visual Basic for this optimization. Many design examples from various applications have been optimized using this program. Several of these examples are presented to demonstrate the efficiency of the algorithm coded in the study.