FRESHWATER Anopheles gambiae may be distinguished as two co-existing biological races1–3, on the basis of their maxillary indices, namely : (i) a paucidentate race, breeding in temporary pools of low organic content, which is anthropophilic and mainly exophilic, and (ii) a multidentate race, breeding in permanent types of water of high organic content, which is zoophilic and end ophilic. Others4–7 hold the opposite view, that the maxillary index is not a precise enough character for demonstrating this distinction. The present observations are in agreement with the reported breeding preferences of paucidentate and multidentate populations, but they also support the conclusion in ref. 6 that the larval environment has a powerful influence on the maxillary dentition.