A new and accurate technique for extracting the parasitic resistance values from GaAs MESFET's for nonlinear circuit models is presented. The technique makes use of only two simple de measurements. Using this extraction method, good agreement between simulated and measured data for the nonlinear Statz MESFET model is obtained, so it is very appropriate for large-signal device and circuit modeling purposes. The presented extraction method is implemented in HP-ICCAP, a software tool for parameter extraction, In addition, all the measurements are controlled by this software, so a high level of automation is obtained.