The acute toxicity of five synthetic detergents to two freshwater teleostsChanna punctatus andSaccobranchus fossilis has been worked out. The LC50 values and slope function (S) for one nonionic and four anionic detergents were determined at 24, 48, 72 and 96 hr intervals by graphical interpolation and probit analysis. Side by side regression equations for all exposure time intervals and fiducial limits for LC50 at the 96 hr exposure were calculated. The relative potency of synthetic detergents was in the decreasing order from nonionic to anionic for both the fishes respectively. The relative susceptibility was highest for nonionic than anionic respectively forChanna punctatus in comparison toSaccobranchus fossilis.