The distribution offish within the Seven Creeks River system, a tributary of the Goulburn River in the Murray–Darling basin, was determined primarily by a survey carried out during the summer of 1975–76. Information on the past occurrence of fish in the system was obtained from historical records. Seventeen species offish, eleven native and six exotic, have been recorded from the system. Habitat characteristics, cohabiting species and food habits are presented for each species recorded during the survey, and factors affecting present distributions are discussed. Siltation appears to have had adverse effects on native Murray cod, Maccullochella peeli, and Macquarie perch, Macquaria australasica, both of which were once common in the lower reaches of the system. The presence of Macquarie perch and the rare native trout cod, Maccullochella macquariensis, in the upper reaches of the system is traced to fish released in 1921 and 1922. A nother native species, the western carp gudgeon, Hypseleotris klunzingeri, was first observed in the system only after it had been introduced into farm dams in the area in the mid 1960s. Relationships between native and introduced fish are complex. Although the food requirements of some native and introduced species overlap and some native fish have been found in the stomachs of introduced species, the only evidence of a substantial effect of an introduced species on a native species is the apparent fragmentation of the range of common mountain galaxiids, Galaxias olidus, by brown trout, Salmo trutta, whose numbers in the system were, until recently, augmented by continual releases of hatchery-reared fish.