Fish species diversity indices (natural bels/individual) calculated for both fish weights and numbers from trawl collections were found to be useful indicators of environmental and pollution stress in Galveston Bay, Texas. Diversity values ranged from 2.1 in the Texas City-Galveston area to 0.02 in the Houston Ship Channel. Thus it is demonstrated that the concept of using species diversity to indicate adverse water quality conditions is applicable to the higher trophic levels of an estuary. Significant differences were detected in diversity between areas of the bay within each sampling period except in winter as well as between seasons. Also, significant differences between the weight and number indices existed, indicating that both biomass and numbers of organisms should be utilized when studying the diversity of higher trophic levels. Correlation of diversity with percent waste water indicated that those areas receiving the greatest amounts of effluents and toxic materials (up to 86% effluent by volume) exhibited the lowest mean annual diversities. Fish diversity in the Houston Ship Channel above Baytown, Texas can be used to predict diversity in the bay because of the linear relationship between distance and dilution of the ship channel effluent (19% effluent by volume calculated for Bolivar Roads. Sampling throughout the system indicated that the fish populations could be divided into somewhat separate communities, each structured as a response to environmental and pollution stress. In those areas receiving the greatest stress, the bay anchovy, Anchoa mitchilli, was the dominant species. These same areas also supported the fewest numbers of large individuals.