Abstract The two biotypes of the mosquito species Culex pipiens s.s. (biotype pipiens and biotype molestus ) differ in overwintering strategy. Biotype pipiens enters diapause, whereas biotype molestus remains active year-round. In the Netherlands, flooded crawlspaces may provide sheltered development sites for biotype molestus in winter. We therefore investigated the effects of human population density and crawlspace flooding on the presence of Cx. pipiens s.s. biotypes and their hybrids using c