Toxicity tests are commonly used as a tool to determine the standards of water quality for chemicals and to discover appropriate organisms as bioindicators in toxicological studies, and also could be used as an essential tool for evaluation of the pollutant effects in aquatic ecosystems. The aim of the present study was to evaluate the sensitivity of two Caspian fish species, Caspian trout as an endangered species and Caspian Kutum using the static acute toxicity in response to nonylphenol, which is widely discharged into the Caspian Sea environment.The fish were exposed to various concentrations of nonylphenol for 96 hours to determine the LC50. The experimental nonylphenol concentrations were consisted of five exposure groups for each species: 100, 150, 200, 250, and 300 ?g/l for Caspian trout; 400, 800, 1200, 1600 and 2000 ?g/l for Caspian Kutum, with a control group for each of them.Using probit analysis, the 96 h LC50 of nonylphenol to Caspian trout and Caspian Kutumwas determined to be 204.78 and 1262.36 ?g/l, respectively. In addition, Caspian trout was approximately 6 times more sensitive than Caspian Kutum. Nonylphenol was reflected to be \"highly toxic\" to Caspian trout and \"moderately toxic\" to Caspian Kutum. The results could be considered in preparing plans for conservation and restocking management of Caspian Kutum and the endangered Caspian trout.