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. . Executive Summary . Information on the fish community of the lower Duchesne River was very limited. This study served to gain additional information regarding the fish community ofthe river. The specific objectives of this study included: I) Document by field sampling the annual distribution and relative abundance of larval or early juvenile Colorado pikeminnow, razorback sucker, other native fishes, and nonnative fishes in the lower 34.5 miles of the Duchesne River; and 2) Determine relations among observed trends, over the course of the study, in distribution and abundance of Colorado pikeminnow, razorback suckers, other native fishes, and nonnative fishes in the lower Duchesne River and the annual and seasonal hydrograph, These objectives were achieved by sampling from May through early September of 1997 through 1999 on the lower 34.5 miles of the Duchesne River. Sampling efforts to detect larval fish were conducted from May through July on the lower 14 miles of river, Three monitoring reaches representing different river channel characteristics were sampled for post-larval fish once every other week from July through September. Endangered species captured during larval sampling efforts included one probable larval razorback sucker and two young-of-the-year Colorado pikeminnow. Larvae of native species captured included flannelmouth sucker, bluehead sucker, speckled dace and roundtail chub, Larvae of red shiner, sand shiner and fathead minnow were the most common. Other species collected during larval sampling included black bullhead catfish, brook stickleback, common carp, channel catfish, green sunfish, smallmouth bass and white sucker. Native species collected during post-larval sampling using seines, included speckled dace, flannelmouth sucker, bluehead sucker, and roundtail chub. Red shiners and fathead minnows were consistently the most numerous species collected in seines and were very abundant throughout all habitats and reaches. Other species captured included redside shiner, black crappie, bluegill sunfish, largemouth bass, Utah chub, and white sucker. The degree of association between flow parameters and relative fish species abundance was estimated using Pearson r correlation. In some instances, magnitude of flow regime among years and the relative abundance of fishes of the lower Duchesne was significantly or strongly associated during the study period. Higher peak flows were associated with lower catch rates of larval red shiner and fathead minnow and higher catch rates of larval flannelmouth sucker, and speckled dace. Although a very small number were caught, catch rates of young- of-year smallmouth bass were lower during the year of sustained high peak flow and sustained high . . . . . . . VI .