Lake Gregory is a large semi-permanent lake system in arid north-western Australia. Its catchment extends intohumid areas and as a result the lake has dried only twice in the last 25 years. Although the system is mostly fresh,parts of it become saline as they dry.We identified aquatic invertebrates and undertook chemical analysis of watersamples from several sites at Lake Gregory in 1989, when the main water-body was saline, and in 1991 and 1993,after the system had flooded and was fresh. During the period 1989–1993, salinities varied from 0.1hto 82h, andionic composition ranged from strong sodium chloride dominance, in saline water and fresh water of the easternpart of the system, to bicarbonate dominance in fresh water of the western area. At least 174 invertebrate specieswere recorded, including two mollusc species that were never collected live. This species richness is much higherthan that recorded from other Australian arid zone lakes, probably owing to long periods of inundation with freshwater. The fauna was dominated by insects (42 per cent of total species richness), crustaceans (27 per cent) androtifers (22 per cent). Most species (160) were restricted to fresh water; only 12 species were found in salinewater. Only one ostracod occurred in saline conditions, although ostracods are a dominant group in Australiansaline lakes. Among species restricted to fresh water, the proportion of rotifer and protozoan fauna that occurred inbicarbonate-dominated water was greater than the proportion of insect, crustacean and hydracarine fauna that didso.