Occurrence of macroinvertebrates and fish was studied at 11 stations along the course of a tropical river, Tamiraparani, South India. Monthly variations in macroinvertebrate density were studied during 1991. Among macroinvertebrates, hemipterans were the dominant group at all sites, except site 2. Using an ordination technique, density and occurrence of macroinvertebrates were correlated with dissolved oxygen, biological oxygen demand and chemical oxygen demand. A total of 36 species of fish was recorded and their diversity in undisturbed and disturbed regions of the river was compared on a spatial scale. Different diversity indices were calculated to assess fish response to pollution and to determine species richness and composition.
