In this letter, a genetic programming method is used to synthesize filters. In order to improve the group delay characteristics, we propose a novel two-stage fitness function reflecting not only the frequency response but also the group delay characteristics of the evolved filters. We also deal with two practical design considerations, i.e., the filters include parasitic effects and are composed of elements with discrete values. The proposed method is applied to low-pass filter design cases. The experimental results show the method can effectively generate filters satisfying the design considerations and possessing improved group delay characteristics when compared with traditional filters.