The two major African malaria vectors Anopheles gambiae sensu stricto and An. arabiensis are sibling species that occupy different climatic niches but are frequently found in the same larval habitats. Differences in survival and development of the aquatic larval stages of these species at different temperatures may help explain adult distribution. The development time from first instar larva to adult at constant water temperatures (25, 30 and 35 degrees C) was measured in these two species when reared together in the same container (ratio 1:1) and separately. Survival to adult was highest in both species reared at 25 degrees C and decreased with increasing temperature. More adult An. gambiae s.s. were produced at 25 degrees C than An. arabiensis (80% interquartile range (78-88) versus 68% (63-78)) but this situation was reversed at 35 degrees C (7% (3-17) versus 33% (27-32)). The survival of An. gambiae s.s. when reared alone was similar to that when reared in the presence of An. arabiensis. In marked contrast An. arabiensis suffered reduced survival when raised with An. gambiae s.s. at 30 degrees C (20% (7-57)) than when reared independently (57% (45-72)). Mean age at eclosion and adult size decreased for both species with increasing water temperature, however An. arabiensis larvae developed at a slower rate and resulted in larger adults than An. gambiae s.s. throughout. The apparent greater production of An. arabiensis at high water temperatures and An. gambiae s.s. at lower water temperatures may in part explain the spatial and temporal distribution of the two species.