Toxicity of water extraction of castor bean shell to bipolaris maydis was measured using plate growth rate method. The regression equation is y=1.941 1x +3.968 8, y=1.706 6x +3.957 8 and y=1.574x +4.090 8 and the EC₅₀ is 3.4, 3.6 and 4.1 mg/ml at 24h, 48h and 96h. The results showed that the effect is very notable and it becomes weak as the time goes.