Two populations of the Purple-crowned Fairywren Malurus coronatus were studied between 1982 and 1987. Both subspecies were represented: M. c. coronatus (Kimberleyan) at Drysdale River Crossing, Western Australia, and M. c. macgillivrayi (Carpentarian) at Riversleigh Station on the Gregory River, Queensland. Using playback of a territorial call, 206 birds were caught and individually colour-banded. At both locations, progeny remained with their parents after reaching maturity and helped to raise siblings; such cooperative breeding is a feature of the genus Malurus. Both males and females called loudly and frequently duetted. Territories were maintained throughout the year and were usually 200-300 m long in linear succession, embracing both banks of the river. Breeding can probably take place throughout the year but was commonest early or late in the dry season (March-September). Nests (25) were large, usually placed at the base of a Pandanus aquaticus leaf 0.25-2.5 m from the ground. Our few data suggest that the clutch size is 2-3, incubation 14 days and nestlings fledge when about ten days old. Competition with other species was slight for these specialist insectivores foraging at ground level or through dense cover, seldom more than 5 m from water. Annual productivity varied from 0.22 to 1.95 yearlings per group depending on season; the mean was 0.78 at Drysdale and 1.04 at Riversleigh. Survival of juveniles and adults was high; a third of yearlings reached four years old and 70% of breeders survived from one breeding season to the next. Our data suggest that the species will be slow to recover from devastation or to colonise vacant areas. They depend on a very specialised habitat that is easily damaged by stock or fire. Loss of this habitat could lead to local extinction of the species and management should focus on maintaining the riverside vegetation as a renewable resource.