This paper addresses frequency assignment for singlelevel and multilevel SCPC systems. In order to obtain the optimum channel allocation, where the influence of intermodulation noise on carriers is minimized, a practical and effective method is proposed. The quasi-optimum, solutions obtained with the proposed method are presented for single-level SCPC systems, showing their advantage in intermodulation noise reduction. Concerning frequency assignment for multilevel SCPC systems, two strategies to realize the-quasi-optimum channel allocation are compared with regard to the improvement in carrier-to-intermodulation noise power ratio. The performance of the obtained channel allocation indicates the effectiveness of the proposed optimization method.