This study proposes a bi-level optimization model for the transit frequency setting problem. A relatively new meta-heuristic algorithm, firefly algorithm is employed in upper-level to determine optimal headways for a given route structure to minimize operator cost. The approach adopted to achieve the objective is to ensure efficient use of vehicles by obtaining maximum occupancy rate. Frequency-based transit assignment is done using PTV VISUM software package in lower-level in order to determine flows resulting from headway set generated in upper-level. Tests are performed on Mandl’s network and transit network of Karaman, Turkey to evaluate the performance and applicability of the proposed model, obtained results are presented, and some conclusions are drawn.