Genetic mechanisms of filarial nematode susceptibility were studied in Aedes togoi. Acid hydrolases may play an important role in this process, including humoral or cell-mediated defenses. Levels of acid phosphatase, alpha-glucosidase, beta-glucuronidase, and N-acetyl-beta-glucosaminidase were determined for 1st- and 4th-instar larvae, male and female pupae, and 1- and 7-day-old adults using fluorometric and colorimetric assays. Acid phosphatase activity was highest in 1-day-old adults, moderate in larvae and pupae, and lowest in 7-day-old adults. Female 7-day-old adults had significantly higher levels than males of the same age. Moderate levels of alpha-glucosidase were found in larvae, with progressive increases in activity from pupae to 7-day-old adults. Levels in male pupae and 1-day-old males were higher than in females, but activity was twice as high in 7-day-old females. Activity of beta-glucuronidase was greater in adults, with females showing a 2-fold higher level than males at 7 days. In contrast, N-acetyl-beta-glucosaminidase activity was highest in 1st- and 4th-instar larvae and 1-day-old males and females. Activity also was significantly higher in male pupae, slightly greater in 1-day-old males, but twice as high in 7-day-old females when compared to males of the same age. Results showed significant changes and variation in acid hydrolase enzyme titers in the different life stages of Ae. togoi. These and other specific acid hydrolases could prove effective in monitoring biochemical and genetic changes in mosquito populations.