Two formulations of the insect growth regulator methoprene were applied aerially (0.1 Ib active ingredient/acre) to rice fields in the Sacramento Valley of California. The level of control of Culex tarsalis was assessed in emergence cages established before and after spraying in these and control fields. In cages established immediately after spraying, treated fields had about one half the rate of Cx. tarsalis emergence as did control fields. The level of control diminished to ca. 10 to 30% reduction by the 4th day after spraying, and by the 7th day emergence rates in treated fields were not significantly different from those exhibited before treatment began. The abundance of various non-target organisms was simultaneously assessed in treated and control fields. None of the organisms examined exhibited population fluctuations which could be statistically attributed to the methoprene applications.