The M and S molecular forms of Anopheles gambiae sensu stricto Giles are thought to be reproductively isolated through premating barriers. However, the exact mechanisms of recognition of conspecific partners are unknown. Because mating in An. gambiae occurs in swarms, one might expect swarming behavior between the M and S forms to be different and that this probably reduces the risk of contact between males and females of the different forms in areas where they are sympatric. We report the occurrence of four mixed swarms, containing males of M and S forms, out of a total of 26 swarms sampled in Soumousso, a typical savannah village of Burkina Faso, West Africa. However, the frequency of mixed swarms was lower than that expected by chance. This observation suggests partial segregation between the swarms of the molecular forms, which may contribute to their isolation. Because the frequency of mixed swarms seems too high to explain the low frequency of cross-mating and hybrids, we suggest that mate recognition in a swarm is more important than swarm segregation.