The effect of pyridalyl on cultured Sf9 cells was compared with that of various known cytotoxic and non-cytotoxic substances. Cell growth was significantly inhibited by the cytotoxic substances at 10 μM, but not significantly inhibited by the non-cytotoxic substances. Pyridalyl clearly inhibited the growth of cells at a concentration of 0.01 μM. Analogues of pyridalyl with insecticidal activity also inhibited cell growth, but those without insecticidal activity did not have significant effects. The test system was considered valid and the cytotoxicity of pyridalyl in Sf9 cells reflects its insecticidal action at least in part. Pyridalyl did not, however, affect mitochondrial respiration, which is the most likely cause of the cytotoxicity. © Pesticide Science Society of Japan