Isozyme profiles of mosquitoes from laboratory colonies of Culex ocossa and Cx. panocossa for 18 enzymes were made and compared. It is possible to differentiate the larvae, pupae and adults from these colonies by their distinct acid phosphatase (ACP), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (adults only), phosphoglucoisomerase (GPI) and zanthine dehydrogenase (XDH) isozyme patterns. The isozymes produced by other enzymes could not be used for specific identification, due to certain similarities in their isozyme migrations. Study of 100 field collected and taxonomically identified males, 86 Cx. ocossa and 14 Cx. panocossa, showed that each male of a particular species' field sample had an isozyme profile for ACP, GAPDH, GPI and XDH that was identical to the profile of the appropriate in colony species. The isozyme profile of these 4 enzymes was used to specifically identify Cx. ocossa and Cx. panocossa collected from a mixed population of the 2 species collected in the Juan Mina are of central Panama. Adults attracted to man bait and larvae (later reared to adults) were collected at 8 sites. These specimens were specifically identified by isozyme analysis, and both larval and adult collections were shown to consist of 80% Cx. ocossa and 20% Cx. panocossa. Neither species had a greater attraction to man bait than would be expected from their species' densities.