The dry-season biology of a member of the Anopheles gambiae complex (probably species B) was studied in 2 areas in the Khartoum region of Sudan. It was found that in the valley of the White Nile the species maintained itself by low-level breeding, as shown by the continuing presence of larvae, male mosquitos and parous females through the dry months (9 months in the year). In the scattered villages of arid areas situated more than 20 km from the Nile Valley, on the other hand, regular sampling through the cool dry and hot dry months of the year failed to detect any An. gambiae except nulliparous females. These were found in occupied huts, deserted huts, dry wells and animal burrows.The great majority of 213 females collected in the 11 dry months between November 1966 and December 1967 had fresh or older blood-meals but the abdomen was never found fully distended in the dry season. Examination of the ovaries showed that they did not develop beyond Christophers' stage II in the period from November to February, stage III in March and April, or beyond stage IV in May. But, in June and July stage IV and V ovaries predominated and few specimens remained in stage late-II.It is inferred from these observations that the local population of An. gambiae is highly adapted to survive in the adult stage through the severe drought and heat of the arid zone of Sudan. Some feeding activity continues but ovarian development is extremely retarded, and only one batch of eggs matures during the whole 9-month period. Evidence collected in the Nile Valley indicated that female An. gambiae in that area were not subjected to similar retardation of the ovarian cycle; in fact, clear evidence was obtained there of continuous year-round breeding by the mosquito.