In the High Capacity Mobile Telephone System (HCMTS) many closely adjacent radio frequency channels are used and for economic reasons it is desirable to multiplex these signals for transmission from a single antenna. The multiplexer used in HCMTS consists of 16 high Q cavity resonators which act as highly selective bandpass filters. These cavity resonators are tied together at their common junction by lengths of transmission line which are three-quarters of a wavelength long at the center frequency of the HCMTS transmit band (880 MHz). The cavity resonators are made of invar to provide temperature stability and are copper plated and gold flashed to provide proper electrical and environmental properties. This multiplexer provides a nominal loss of 3 dB from any input to the output and an isolation of about 20 dB between any input sources. A photograph of a complete multiplexer is shown in Figure 1. Because of the complex nature of this multiplexer and the significant cost of building prototype models, it is most desirable to develop a method for simulating the characteristics of the multiplexer. There are many applications where simulated results are useful including the determination of the optimum loaded Q for the individual cavity resonators, the determination of heat dissipation throughout the device and the study of changes due to temperature variations. A computer simulation for the multiplexer has been developed and will be described along with results in the above areas.