A simple relationship between the interface trap densities and transconductance of depletion mode field-effect transistors is derived without any simplifying assumptions regarding the energy distribution of the traps. Using this relationship and the experimental transconductance data, interface trap densities in GaAs and InP were calculated and compared to their values obtained using capacitance versus voltage measurements. Our method provides a simple and efficient technique of estimating interface trap densities based on transconductance data that are readily given in the pertinent literature.