AbstractIn order to understand how the malaria vector Anopheles arabiensis Patton exists in the extremely hot and arid environment (mean annual rainfall 21 mm) in Northern Province, Sudan, regular sampling from breeding places and resting sites was undertaken during 1978 and 1979 at four villages along the River Nile downstream (i.e. northwards) from Dongola. The population density of A. arabiensis was lowest during the flood season (July—October) and increased as the river flow decreased during November—June. Thus there was an inverse relationship between the Nile water level and A. arabiensis production. Prolific breeding in riverside pools was the main source of A. arabiensis as the river receded. During the annual flood, when riverside pools were all inundated, A. arabiensis continued breeding at low densities in sheltered sites such as wells and pits. Breeding also occurred in all months in association with the weed Potamogeton crispus in slow reaches of the river. Adult females fed regularly on blood, with gonotrophic concordance in all months, and there was no evidence of aestivation. Outdoor biting activity occurred throughout the night, with a peak between 2100 and 0500 h. Daytime searches indoors and outdoors indicated that resting females A. arabiensis are almost entirely endophilic in the study area.