The cytotoxic effects of Bacillus thuringiensis (Bt) $delta$-endotoxin, a well-known bioinsecticide, were investigated on the ion channels of insect cell lines. This study attempted to evaluate the effect of Bt toxin on lepidopteran cell lines by measuring the cell swelling in isotonic solution containing only one cation. Cell swelling was stimulated in KCl-sucrose isotonic solution as well as TC-100 media containing solubilized crystal $delta$-endotoxin. It suggested that cell swelling by Bt toxin is related to a $K^{+}$ channel. Cell swelling may be due to the stimulation $K^{+}$ influx and simultaneously the penetration of $H_2O$ induced by Bt toxin, because the stimulation of swelling was observed with solubilized toxin in KCl-sucrose isotonic solution, but not in sucrose isotonic solution. Moreover specific $K^{+}$ channel blockers, such as 4-aminopyrimidine (4-AP) and ouabain, showed the significant effect on cell swelling induced by Bt toxin. We suggest that the increased cell swelling induced by 4-AP is caused by blocking of $K^{+}$ efflux through $K^{+}$ leaky channels. The inhibition of cell swelling by ouabain, which is a well- known inhibitor of $Na^{+}$, $K^{+}$-ATPase, is suggested to be due to decreased $K^{+}$ influx following diminishment of $Na^{+}$, $K^{+}$-ATPase activities.