To explore the southern limit of the temperate zone form of the Culex pipiens complex of mosquitoes in North Africa, we compared the structure of the copulatory structures of such male mosquitoes sampled along the length of Egypt. Larval mosquitoes were collected in 11 sites distributed along the entire Egyptian span of the Nile Valley as well as in sites along the central Red Sea coast and in the western desert. Four of these sites were sampled repeatedly, at various times of the year. A DV/D ratio was calculated for each adult male mosquito. Although all DV/D values conformed to criteria normally attributed to the temperate zone form, even at the southernmost site (22 degrees N), values tended toward those of the tropical form in southern sites and during the summer months. We concluded that Cx. pipiens mosquitoes of the Middle East are predominantly Mediterranean in form due to an isolating effect of the Sahara Desert which separates these temperate zone insects from those of tropical Africa.