Excellent control of larvae of Culex tarsalis in experimental ponds was obtained at the rate of 1 tossit/3 m2 (44 g AI/ha). At the same rate, satisfactory control (92%) of Cx. Peus larvae was obtained in dairy waste-water lagoons in Riverside County, Calif. At the effective mosquito larvicidal rate (44 g/ha), mosquito fish, Gambusia affinis, was affected when some tossits were placed with the fish in 0.9 m3 screened cages. Five successive weekly treatments of G. affinis at the larvicidal rate and double that rate (44 and 88 h/ha) resulted in 37 and 67 percent reduction of fish yield harvested 42 days after the first treatment compared to the yield obtained from the untreated ponds (checks). No marked effects on some of the nontarget organisms such as dragonfly naiads, ostracoda, copepods, and Dixa midge larvae. Mayfly naiads were eliminated at all rates applied, but recovered within 2-3 weeks after treatment.