Summary Several colonies of Anopheles gambiae were established in the laboratory. Egg production by colony bred females was considerably less than that of wild females, the former averaging 94 and the latter 192 eggs per batch. Oviposition occurred throughout the night but most frequently during the hours before midnight. No difference was observed in the hatching rate of eggs from wild and laboratory bred females. In a series of experiments in the laboratory involving more than 13,000 eggs the duration of the period between oviposition and pupation varied between 7 and 27 days. When kept under favorable conditions larvae were brought through to pupation in 4 days and 22 hours. The shortest pupal stage observed was 19 hours. Anopheles gambiae was shown to be highly domestic by the fact of its presence in occupied houses and rooms used as sleeping quarters, and by its relative absence from unoccupied houses and rooms not slept in by man. A direct correlation was demonstrated between the number of Anopheles gambiae discovered and the darkness of any given room. Migration from houses situated in a moist region was less than from houses in a hot dry region. In the former, however, more than four-fifths of the mosquito population in any given house changed within 24 hours after observations were begun. Experiments using both man and animal bait inside and outside of houses revealed that Anopheles gambiae feeds only inside houses and that man constitutes the preferred host. Dissections on the three species of Anopheles prevalent in the experimental areas showed Anopheles gambiae to be the principal vector of malaria with an infection rate of 5.6 per cent as compared with rates of 1.5 and 0 for a species of the tarsimaculatus series and Anopheles albitarsis respectively. The gametocyte rate of the human population varied between 9.3 per cent and 20.9 per cent, during the period when dissections were made. Sixty-six laboratory bred Anopheles gambiae dissected after feeding on five human gametocyte carriers showed an infection rate of 9.1 per cent. Mosquitoes fed on three of the carriers failed to show oocysts while the stomach infection rate of specimens fed on the other two cases were 33.3 and 4.2 per cent. Preferred breeding places of Anopheles gambiae were found to be small collections of fresh water, comparatively free of vegetation, fully exposed to the sun, and usually near human habitation. Such collections were available in northeast Brazil throughout the year. Although gambiae larvae were never found in brackish water in nature, laboratory experiments showed that development could take place up to one per cent sodium chloride concentration.