A survey of alcohol dehydrogenase (Adh) in Culex tritaeniorhynchus Giles has uncovered five electrophoretic variants among laboratory strains. Detailed genetic analysis revealed that these variants are inherited as codominant alleles at a single locus. The Adh locus has been assigned to linkage group III between s and cl. A survey of a natural population from Lahore, Pakistan, revealed the presence of four of the five variants and suggests the possibility of using this enzyme polymorphism to monitor a release programme. Substrate specificity tests suggest that this enzyme can utilize almost any aliphatic primary alcohol as a substrate.