Abstract: A static bioassay method to determine the acute toxicity of the herbicide propanil to Oreochromis niloticus was investigated under laboratory conditions for 96 h exposure period. Concentrations of propanil used include: 0.0096, 0.0103, 0.011 and 0.012 ml L-1, respectively. The lethal concentration (LC50) value of propanil on fingerlings of Oreochromis niloticus was 0.0097 ml L-1 for 96 h of exposure. The regression equation for probit kill from propanil was found to be Y = 29.84+12.33xlog conc. (R2 = 0.9951, Y = Probit kill). Fish showed various abnormal behaviors upon exposure to propanil. Immediate reaction was erratic swimming and tendency to jump out of the test bowl. Others include restlessness, un-coordinated movement, vertical swimming, air gulping, equilibrium loss, a period of quiescence and eventually death.