Six insect growth regulators (IGRs) were screened in the laboratory against 4th instar larvae of Culex quinquefasciatus and Culex incidens. Two of the urea type IGRs Bay SIR-6874 and SIR-8514 showed high activity (LC90 about 1-7 ppb) against both species, the former being about 3-4 times as active as the latter. However, since further development of the former was stopped, the latter was studied in the field. The JH type IGRs MV-678 and CGA-19255 were less active, the LC90 of MV-678 in the range of 5-70 ppb and that of CGA about 1 ppm. The latter species was the most susceptible. Pupae were not susceptible to these materials. EC, WP and G (granular) formulations of Bay SIR-8514 and EC and flowable formulations of MV-678 were evaluated in the field against flood and stagnant-water mosquitoes. Psorophora columbiae was the most susceptible, Aedes nigromaculis next in susceptibility while Cx. tarsalis required the highest rate of application for effective control. Bay SIR-8514 and MV-678 were equally effective in the range of 0.01-0.025 Ib/A against Ps. Columbiae. Against Ae. nigromaculis the former compound was effective at 0.01, but MV-678 was effective in the range of 0.05-0.1 Ib/A. These same rates of both materials were also needed for effective control of Cx. tarsalis. MV-678 produced excellent control for 1 week only, while Bay SIR-8514 prevented adult emergence for more than 2 weeks.