Growth of insect cells in vitro was inhibited by addition of rotenone to culture media. The rate of inhibition did not vary in cell lines originating from the same tissue of the same species. However, there were great differences between cell lines derived from different species or different tissues. Among the tested cell lines, the ones from Mamestra brassicae ovaries were most sensitive and the ones from M. brassicae hemocytes were most insensitive to rotenone. Respiratory rate did not vary in cell lines from the same species regardless of the original tissues. However, it varied considerably between cell lines from different species. The flucturation in rates of respiratory inhibition by rotenone showed a tendency similar to that in the rates of growth inhibition.