An arylterpenoid insect growth regulator MV-678(2-methoxy-9-[4-isopropylphenyl]-2,6-dimethynonane) was tested against 6 species of mosquitoes from Malaysia. When assayed at early 4th instar larval stage, LC50 values for Aedes aegypti (Linnaeus), Ae. albopictus (Skuse), Culex quinquefasciatus (Say), Cx. pseudovishnui (Colless) and Cx. tritaeniorhynchus (Giles) were 1.73, 1.86, 0.22, 0.15 and 0.01 ppb respectively. Armigeres durhami (Edwards) was the least susceptible with LC50 value of 5.64 ppm. In the same tests, mortality of Aedes and Armigeres species occurred mostly at the pupal stage, whereas death occurred more often at the larval stage for the Culex species. The susceptibility of mosquito immatures to MV-678 varied with age. The 1st and 2nd instar larvae of Ae. aegypti were less susceptible than late instar larvae with the pupal stage being the least susceptible. Besides causing mortality, MV-678 also delayed the adult emergence of 3 of the mosquito species tested.