Prudent management of lotic systems requires information on their ecological status that can be estimated by monitoring water quality and biodiversity attributes. To understand the environmental conditions in Ban Ganga stream (Katra), a study was carried out to establish the relationship between water quality and macroinvertebrate assemblages between the months of Jan. and Dec., 2003. Water physico-chemical characteristics (temperature, depth, speed, pH, DO, FCO₂, CO₃", HCO₃", Ca⁺⁺, Mg⁺⁺ and Cl^-) and biodiversity index changed markedly along the longitudinal profile of stream. In total, 13 species of phylum Annelida, Arthropoda and Mollusca were recorded. The highest number of species (12) were observed at S-I and lowest number (10) at S-III. The order of dominance of various groups was as : Arthropoda > Annelida > Mollusca. Moreover, the highest species diversity index of 5.157 was recorded at S-II and the lowest 4.151 at S-III. Thus, the present study indicated that significant changes in aquatic macroinvertebrate assemblages were primarily due to water quality rather than prevailing climatic conditions.