In a longitudinal study we have observed the fluctuation in density of two Anopheles populations responsible for malaria transmission in a village built on a brackish lake. The two A. gambiae complex species are A. melas and A. gambiae s.s. The former is the most abundant (88%). The brackish lake ecosystem gives to both species possibilities of adaptation. The salt water species of the A. gambiae complex, A. melas, was found to be tolerant to waters with very low salinity. On the other hand, A. gambiae s.s. was found to support relatively high salinity rates. Both specimens live together the whole year round but their frequencies vary with the inundation of the lake. During inundation the salt rate decreases and A. melas eventually disappears. Transmission is low, seasonal and short on the lake. It is perceptible between March and August. The inoculation rate (11 infected bites per man/year) is lower than what we have observed in other lagoon areas of Benin, or even in the city of Cotonou (33 infected bites per man/year). The low malaria transmission on the lake is probably due to the presence of an important population of A. melas with low infection rates and the widespread use of bed nets.