The only mosquitoes found to be concerned in malaria transmission were Anopheles funestus, Anopheles costalis and Anopheles pretoriensis. The natural index of infection in each case was: funestus, 15.9 per cent (340 examined); costalis, 15.6 per cent (205 examined), and pretoriensis, 1.7 per cent (114 examined). The conditions were found to be favorable to species control of the incriminated anophelines. A. funestus breeds in streams with grass-covered banks, especially where the current is not strong; it prefers water that is clear and fresh. A. costalis is a puddle breeder, being found in the most temporary mud puddles which are lacking in vegetation and without shade. A. pretoriensis is thought to play little part in natural transmission, probably because of its lack of contact with man. Evidence is presented which indicates that human gamete carriers (of estivo-autumnal malaria) with infections as low as 1 gamete to 1,500 leukocytes may infect anophelines.