The Western Ghats of India is one of the major biodiversity hot spot regions of the world containing 84 endemic fauna. With reference to biodiversity, aquatic fresh water ecosystem has been widely neglected though it contains a tremendous diversity of fishes, amphibians, invertebrates and aquatic plants which are still the mostly serious threatened (global biodiversity strategy, 1992). In India, the National Bureau of Fish Genetics Resource, Lucknow and Zoo Outreach Organization have recently evaluated 329 fresh water species (ZOO/CBSG India, 1998). Peninsular India with special reference to Western Ghats has 64 ornamental fishes (GopalaKrishnan & Ponniah, 1998). Despite this rich biodiversity some of the species are even endangered due to various anthropogenic factors (Cairnsetal, 1993). In this context River Cauvery, one of the perennial rivers of Tamilnadu flows west to east with a number of distributories and tributaries before confluencing the Bay of Bengal. In Tamilnadu its major reservoirs are Mettur and Grand Anicut. K Jayaram et al. (1982) published a survey of the entire River Cauvery system with a major account on fish fauna. This micro level study taken out to make an inventory of the fish diversity in and around (5km radius) of Grand Anicut area, Tiruchirapalli. This major reservoir was particularly chosen since at this point the river branches off into five major distributories.