Abstract.-An index of biotic integrity (IBI), which is based on fish community structure, was used to evaluate environmental quality at 22 sites on the severely degraded River Rouge system in southeast Michigan. The IBI scores generally decreased from upstream to downstream sites. The IBI scores appeared to reflect the negative impacts of accentuated flood peaks and low flows, along with increased loadings of pollutants such as oils, on environmental suitability for fish. The IBI appeared to be sensitive to differences in both physical and chemical habitat conditions. With minor modifications the IBI could be used on other Michigan warmwater rivers and would prove useful for tracking long-term trends in the well-being of these rivers.