The esterase system of populations of Anopheles triannulatus from the Balbina hydroeletric project were studied by polycrilamide gel electrophoresis, using α-naphtyl proprionate as enzyme substrate. The analysis revealed six zones of activity, and the mechanism of inheritance was studied for five polymorphic loci. Two aieles were detected for esterases 1, 2 and 4, and three for esterases 3 and 5, all being co-dominant. Genetic frequency data were different for the aleles at each locus, chisquared values for genotypic frequencies indicated that the population is not in equilibrium for all the loci, as homozygote frequencies were higher than expected. This was interpreted as being the result of environmental changes occuring in the study area.