Four insect growth regulators were evaluated in the laboratory and field. In the laboratory, AC-291898 showed excellent activity, inducing 90% mortality in Culex quinquefasciatus and Aedes aegypti at 0.5-0.7 ppb. The EC formulation of XRD-473 was slightly more active than the technical material with an LC90 of 0.84 ppb and 0.92 ppb against Ae. aegypti and Cx. quinquefasciatus, respectively. In the field, AC-291898 at the rates of 0.005 and 0.01 lb AI/acre caused 85 and 100% inhibition of adult emergence of Cx. tarsalis 2 days after treatment. Activity, however, declined at the low rate, while the high rate remained active for more than 7 days. In the same species, XRD-473 induced complete inhibition of adult emergence 2 days after treatment at the rates of 0.01, 0.025 and 0.05 lb AI/acre. At the low rate (0.01 lb AI/acre), activity declined markedly, while the 2 higher rates remained active for one week. In Cx. peus larvae, AC-291898 at the rates of 0.005, 0.01 and 0.025 lb AI/acre produced complete inhibition of adult emergence 2 days after treatment, but activity declined at the 2 lower rates 7 days after treatment, while the high rate (0.025 lb AI/acre) remained active for more than one week. Methoprene (4%) pellets were effective against Cx. tarsalis for 7 days at the rates of 0.25 lb AI/acre whereas 0.5 lb AI/acre was required to obtain similar results against Cx. peus larvae.(ABSTRACT TRUNCATED AT 250 WORDS)