The southwest of Western Australia contains a depauparate, but highly endemic freshwater fish fauna. A major environmental problem impacting on freshwater fishes in this region is secondary salinisation, resulting from land clearing in the upper catchments. Although salinisation has severely restricted the distribution of a number of species of freshwater fish, some, such as the Western Minnow, Galaxias occidentalis, are more salt tolerant and retain a wide distribution throughout the south west. We know very little, however, about the effects of increased salinity on the biology of these salt tolerant species. We studied a number of biological parameters of populations of G. occidentalis in fresh, brackish and saline reaches of the Blackwood River in the south west of Western Australia. Salinity appeared to have significant effects on the size distribution and diet of G. occidentalis.