A novel design procedure for diplexers built in waveguides below cutoff is presented. The design permits the integral construction of a diplexer--the whole unit is built in a single waveguide, thereby dispensing with the T junction and connecting flanges--if coaxial termination is used at the common junction. The design utilizes foreshortened bandpass filters, and is valid for bandwidths of up to a few percent. Simple expressions on calculating the connecting lengths are arrived at. A satellite telemetry diplexer designed using the derived expressions yields results which are in agreement with computed values.