This study attempts to determine the 96-h LC50 Values of CdSO4 . H2O, a metal salt widely used in industry, for grass carp (Ctenopharyngodon idellus Val., 1844). The study was carried out in two stages with 120 grass carp individuals using statistic method of aquatic toxicity. In addition, the behavioral changes of grass carp at different CdSO4 . H2O concentrations were determined. The results were evaluated using probit analysis method and the LC50 value for grass carp species was calculated as 9.42 mg/l. (C) 2003 Elsevier Ltd. All rights reserved.