A thorough, repetitive, 21-month search for breeding sites of Anopheles dirus (=An. balabacensis s.l.) was conducted within a 3-km radius of an isolated, holoendemic malaria focus. During the annual 6-month monsoon, all breeding within 1.5 km of the focus took place at eight sites in 1975 and at nine sites in 1976. These sites were of two topographic types--puddles on footpaths and turbulence pits at the heads of drainage gullies--that shared the characteristic of being able to hold water for at least 5 days without supplemental rainfall. The only dry season breeding was in a stream 3 km from the focus. The effective flight range in forest of An. dirus was deduced to be about 1.5 km. Immatures were found to be peculiarly adapted to, and possibly dependent on, small, temporary pools. Eggs were deposited above the water line where they could remain viable for 2 weeks. Heavy rain produced a synchronous hatch that completed immature development in 5-6 days and could eventually produce waves of parous females biting man. Larvae were observed to leave a draining pool before it completedly emptied and to crawl overland as far as 53 cm, sometimes to another pool. Larvae stranded by receding water often escaped predation that eliminated cohabiting species. They could be recovered alive from drained pool bottoms up to 94 hours after being stranded. Fourth instar An. dirus are passively predatory of first and second instar mosquito larvae. The limited number and topographic distinctness of the habitat of immature An. dirus in Bangladesh may make larviciding an effective control measure.