Roots of Allium cepa (Var. Giza 6) were treated for different periods with solutions of pure and formulated Sevin prepared at 22[degree] C and 60[degree] C. The end-effect of both forms of the insecticide depends mainly on the temperature at which the solutions are prepared. The observed differences in the biological effects of the solutions are attributed to partial hydrolysis of the material under the influence of high temperatures. Solutions prepared at 22[degree] C tend to increase the rate of mitosis; while those prepared at 60[degree] C tend to depress the rate. Continuous treatment with the insecticide for 24 hours nearly stopped mitosis. The roots recovered, however, after replacement in water for 24 hours, and rate of mitosis became normal again.