The time spent submerged during the larval alarm reaction was measured for several strains of Aedes aegypti and Ae. albopictus from different locations. Differences between the 2 species were not significant. Differences in strains within the species were highly significant. Greater differences were seen among strains of Ae. albopictus than among strains of Ae. aegypti. These differences represent an additional indication of extensive local differentiation in Ae. albopictus.