Allozyme electrophoresis was used to examine patterns of genetic differentiation in the western minnow, Galaxias occidentalis, from the North Dandalup and Canning Rivers in south-westem Australia. Two distinct genetic forms of this species were identified in these rivers and both forms occur sympatrically in samples from two sites. A significant deficit of heterozygotes and a non-random association of alleles among loci was observed in these two samples. The distribution of one of the genetic forms extended from the headwater streams in the Darling Range to the transition zone between the ranges and the Swan Coastal Plain. With the exception of one population, the distribution of the other form extended from the transition zone to the Swan Coastal Plain. The exception was a population that was caught from a site in the ranges but grouped with the populations from the coastal plain. Discriminant analyses of meristic and morphological data were undertaken to determine if morphological differences exist between the two identified genetic groups. All individuals were correctly classified by the analyses of samples from each catchment separately and of the two sympatric populations separately. However, the discriminant analysis of individuals from all sites correctly classified only 85.7% of the Darling Range genetic group and 79.4% of the Swan Coastal Plain genetic group. Further research is required to clarify the taxonomic status of these two genetic forms of G. occidentalis.