Esterase zymograms were prepared from 14 species and strains of Culicide mosquitoes by use of starch and polyacrylamide electrophoresis. The zymograms proved to be species- and strain-specific. Variations in the zymograms of individual mosquitos were demonstrated by use of the microgel technique. This technique is described. Sexual dimorphism was shown to occur in the zymograms of one species, Anopheles stephensi. The isozymes were characterized by their reactivity toward different substrates and inhibitors. Their distribution in the mosquito's organism was studied with the whole mount technique and with organ zymograms. The stomach is particularly rich in esterase isozymes. There is a close correlation of these esterases and the lipids known to occur in the stomach epithelium and the esterases may play a role in the digestion of the blood meal. Observations on the effect of storage of samples, at low temperature and while dried on the activity of some isozymes, are reported. It is also shown that certain isozymes apparently are activated by inhibitors.