We used abundance data from fish-population studies (1962–2010) on Otter Creek at Markle's Dam, Vigo County, IN, to examine temporal variation in stream-fish assemblages. Species-richness variation showed a declining trend during the 50 year study period with a mean of 49 ± 8.8 species per decade (range = 39–62 species). Cumulative study species richness comprised 76 fish species with a mean of 21.3 ± 6.1 species collected per sampling event. Dominant species, based on relative abundance during the 50 year period, included Cyprinella spiloptera (Spotfin Shiner), Pimephales notatus (Bluntnose Minnow), Hybognathus nuchalis (Mississippi Silvery Minnow), and Luxilus chrysocephalus (Striped Shiner). Relative abundance of Spotfin Shiner showed an increasing trend during the 50-year study period, whereas Bluntnose Minnow relative abundance showed a decreasing trend. Cumulative frequency-distribution of species richness normalized by decade showed no significant difference, with the exception of the 1970s, which showed a steep decline in the number of species. Repeated-measures analysis of variance showed no significant differences in species diversity (H') or evenness (Pielou's J) over the 5-decade study period. Eleven cool-water species including Chrosomus erythrogaster (Southern Redbelly Dace), Perca flavescens (Yellow Perch), Catostomus commersonii (White Sucker), and Ambloplites rupestris (Rock Bass), and lithophilic spawning species such as Nocomis micropogon (River Chub) and Hybopsis amblops (Bigeye Chub) were extirpated during the study period. Potential changes in sedimentation and thermal gradients may have contributed to these observed extirpations.