Data obtained from cellulose acetate electrophoresis indicate that there are at least 5 separate loci for esterase (EST) activity in the Gorgas Memorial laboratory colony of Culex (Melanoconion) erraticus. Locus EST-1 has a single band of activity which is not present in larvae. Locus EST-2 also has a single band which is present in larvae and pupae but is lost about 5 days after emergence. Locus EST-3 has three alleles, locus EST-4 two alleles, and locus EST-5 three alleles.