The impact of small (fork length < 100 mm) and large (≥ 100 mm) brown trout (Salmo truttaL.) on the distribution of the common river galaxias (Galaxias vulgarisStokell) was investigated in the Shag River (South Island, New Zealand). Riffle reaches were sampled by electrofishing and their macrohabitat was described in terms of substratum, flow, elevation, and vegetation characteristics. Galaxias densities in reaches containing large trout were lower than reaches with only small trout or no trout and trout size was the most important variable determining galaxias density. Densities of galaxias < 80 mm were reduced in the presence of large trout, but not galaxias > 80 mm. Reaches with large, small, or no trout varied in terms of water depth, percentage cobble, and elevation. However, these differences were unlikely to account for variations in galaxias densities because selection for these particular habitat features accounted for only a small proportion of the observed variation in galaxias density. A change in macrohabitat use by galaxias from fast current velocities in sites without trout to slower velocities in sites with large trout may be explained by competition for areas of high velocity (potentially the better feeding areas). However, a combination of interspecific competition and predation by large brown trout is likely to be responsible for the observed reductions in the density of smaller galaxias.