Abstract: Impact of guano deposition by water- birds at vedanthangal water -bird Sanctuary were studied. Chemical composition of water birds guano showed that it contained 29 mg/g(dry weight) of No2 and No3 nitrogen and 89.8mg/g(dry weight) of P2O5.Guano, dissolved in tank water contained 1.73 mg/l of nitrogen and 4.40mg/l of P2O5.Vedanthangal tank irrigated ayacut soil contained 206 Kg of nitrogen ;130 Kg of P2O5 and 97 Kg of K2O per hectare.Nitorgen and P2O5content of vedanthangal tank -water and tank anicut soil showed a significantly higher level when compared to neighboring tank water and tank ayacut soil. The quantum of benefit received by Vedanthangal ayacut farmers due to guano deposit in was assessed. There was also a fertility gradient noticed in vedanthangal tank ayacut.