Selective pressures in laboratory rearing may account for the poor field mating of laboratory reared Culex tarsalis males. Previous studies of swarming behavior of field collected Cx. tarsalis had to be done in the field since such adults did not exhibit normal swarming in cages. Field collected individuals did not swarm normally nor mate effectively in cages. Normal swarming behavior by field collected mosquitoes, subsequent mating, and insemination were observed in a cage modified from a design by Marchand (1985). The use of such a cage could reduce one type of selective mating pressure involved in the colonization of mosquito species.