We examined the importance of zonation and species additions in explaining longitudinal changes in the fish assemblage of a Rocky Mountain stream that descends onto the Great Plains of Wyoming. Community changes along an elevational gradient from 2,234 to 1,230 m above mean sea level reflected a combination of zonation and downstream addition of species. Zonation was evident on a broad spatial scale as a result of stream temperatures. A coldwater trout (Salmonidae) assemblage dominated headwater reaches but was replaced by a warmwater minnow–sucker (Cyprinidae–Catostomidae) assemblage below 2,000 m. Within the warmwater zone, fish community change was due mainly to the addition of new species downstream. Headwater sites were dominated by members of the insectivore feeding guild, and other trophic guilds were added downstream. The major gradient of habitat change downstream consisted of a decrease in pool habitat and increases in stream width, depth, current velocity, turbidity, and proportion of the channel consisting of run habitat. Minor gradients of habitat change involved streambank condition and substrate particle size. Contrary to streams in forested regions, habitat diversity did not increase downstream, suggesting that increased living space and moderating environmental conditions contributed to the downstream increase in species richness. Local habitat modification due to cattle grazing or alterations in streamflow caused minor changes in fish assemblages but did not disrupt the dominant longitudinal patterns. Broad-scale zonation based on temperature regime and additive patterns within zones should typify other streams originating in montane regions