Bunyamwera virus and snowshoe hare virus (family Bunyaviridae) were passaged up to six times through mosquito ceIls in culture and the resultant viruses were compared to the input, mammalian cell-passed virus using monoclonal antibodies raised against the input virus. The mosquito celle-adapted virus population consisted of mutants which were better the input virus. The mosquito cell-adapted virus population consisted of mutants which were better adapted to replication in the new host than was the input mammalian cell- passed virus and were differentially susceptible to neutralization by antibody.