Abstract
Assortative mating in a mixed population of Culex fatigans and C. molestus established in an outdoor population cage incorporating a simulated septic tank is examined using different electrophoretic alleles at the larval alcohol dehydrogenase (Adh) locus, and the larval colour mutant “yellow larva” as markers. One egg raft out of 66 sampled over a period of 13 weeks was the result of an interspecific mating. Observations on C. fatigans and C. molestus populations established separately in the population cage indicate that the mate recognition system is not the only mechanism by which the genetic integrity of the two species is being maintained.