A spatially intensive survey in 1989 of 52 sites in the Red River drainage in southwest Oklahoma and surveys in all years from 1978 to 1987 on four sites in the drainage provided evidence that construction of Altus Dam on the North Fork of the Red River caused major changes in fish community structure in the river above the dam. Pre-impoundment data on the fish communities were scanty, but the inferences they allowed were similar to those obtained by comparing fish assemblages in the North Fork above the dam with assemblages elsewhere in the drainage, particularly along Salt Fork, which had similar habitat characteristics. Twenty-five species were collected in the North Fork above Altus Dam, compared to 33 in the Salt Fork and 34 in the North Fork below the dam. The speckled chubMacrhybopsis(formerlyHybopsis)aestivalisand the chub shinerNotropis potteriwere absent in the North Fork above Altus Dam but fairly common in similar streams elsewhere in the area. The plains minnowHybognathus placitusand the Red River shinerNotropis bairdiwere among the most common fish species found in southwest Oklahoma, but were not collected above Altus Dam in the 1989 survey and were collected only intermittently and in small numbers in the long-term survey. We speculate that these two species have repeatedly been extirpated and have been reestablished as bait-bucket introductions since the dam was closed. Upstream of the reservoir, the sand shinerNotropis stramineusand the emerald shinerNotropis atherinoidesreplaced the plains minnow and the Red River shiner as dominant species, and several reservoir species were more common. Significant negative association at two long-term sites suggested that the sand shiner and Red River shiner were filling similar niches.