A 30-month study of the comparative dynamics of the fish populations inhabiting Kelsey Creek, located in the City of Bellevue, Washington, and a nearby pristine control stream suggest that urban development has resulted in a restructuring of the fish community. Environmental perturbations, including habitat alteration, increased nutrient loading, and degradation of the intragravel environment appeared to have a greater impact on coho salmon Oncorhynchus kisutch and nonsalmonid fish species than on cutthroat trout Salmo clarki. Although the total biomass (g/m2) of fish in each stream was similar, its composition differed markedly. Ages 0 and I cutthroat trout were the majority of the fish community inhabiting Kelsey Creek, whereas the control stream supported a diverse assemblage of salmonids of various ages and numerous nonsalmonids. The rapid growth and greater biomass of salmonids in Kelsey Creek (a 2-year mean of 3.51 g/m2versus 2.03 g/m2in the control stream) resulted in a total annual net production of these species of 1.6 to 3.3 times that of the control stream (a 2-year mean of 7.6 g/m2versus 3.5 g/m2in the control stream). Marking and outmigrant studies indicated that environmental disruptions in the urban stream do not result in the displacement of the salmonid inhabitants.