The installation of ultrasonic units for rodent control ( Scat ) units markedly reduced general rodent activity at a rice elevator during a two-week test period. A greater change in rat than mouse movements during the observation period was noted. There was a 65% decrease in rat movements through critical areas. In some areas rodent activity persisted. This was related to well- habituated activity patterns where alternative routes were not readily available. Mice are known to be more persistent than rats under conditions of high frequency sound stress. Extrapolation from a single experience or data set is not desirable. In the case of this test the rodent population was of low density, and the peripheral pressure was not great. At high population densities, these results are not directly applicable.