The effectiveness of the threespine stickleback as a mosquito control agent was compared to that of the mosquitofish in 28-m2 earthen ponds during 26-wk experiments where the 2 fish were stocked alone and together. Relative to ponds without fish, the stickleback was not effective for controlling larval mosquito populations; however, sticklebacks reduced the abundance of Culex pupae. Mosquitofish provided significant levels of control whether stocked alone or concurrently with the stickleback. As compared to mosquitofish alone, mosquito control was not significantly enhanced when both fish were stocked together. Mortality of adult sticklebacks was related to a gradient of increasing water temperature across the ponds rather than the direct effects of other abiotic factors such as low dissolved oxygen concentrations or biotic interactions with the mosquitofish. The stickleback exhibited a lower thermal tolerance and slower population recruitment as compared to the mosquitofish populations,which reproduced successfully in water > 33 degrees C and grew rapidly. Stickleback biomass either declined or increased slightly (approximately 50% of initial stocking weight). Mosquitofish biomass increased 33- to 38-fold at rates averaging between 0.079 and 0.095 g wet weight/g/day and total wet weight per pond at 6 wk after stocking did not differ significantly between the 2 mosquitofish treatments.