Abstract
A monthly survey of resistance gene frequencies was conducted in a larval population of Culex pipiens breeding in an hypogeous site (''molestus'' form) between July 1989 and September 1991, after interrupting mosquito control. Several significant variations in resistance gene frequency were observed. Some variations are explained by fitness differences between resistant and susceptible insects, others by an immigration of insects from the surrounding epigeous populations (''pipiens'' form), which are highly resistant.