An entomological survey based on collections of human bait mosquitoes and of mosquito larvae was carried out through one year in a rice-field, the Kou Valley of southwest Burkina Faso. Each year, in this irrigated rice field there are two crops of rice, one during the dry season and another during the wet one. Between the two rice cycles irrigation is interrupted and the rice field becomes dry. The rice crop cycle moves through several stages, all of which have positive or negative effects on the development of the larvae of most abundant mosquitoes species. Rice cultivation does not explain however entirely the ecology of mosquito populations. Two main limiting factors for mosquito development are highlighted: the season with two periods (1. dry, 2. wet) and the rice cultivation with three periods (1. start of the flooding, growing rice transplantation and tillering, 2. rice heading and flowering, 3. rice maturation and harvest). The development of a majority of mosquito species depends strictly on rice growth: (i) Anopheles gambiae s.1. uses the whole rice field during the first period of rice cultivation until growing rice protects the larval breeding places from solar radiation. It is the most frequent mosquito (53% of the man-biting mosquitoes caught). 5% of its larvae are parasitized with a fungus Coelomomyces sp., (ii) A. pharoensis has its maximum density during the second period of rice cultivation, as Culex gr. decens; both seem to succeed better in the dry season, contrary to all other anophelines, (iii) A. coustani profits by the third period when the pH of the breeding places becomes basic.(ABSTRACT TRUNCATED AT 250 WORDS)