Cidial, an organophosphorous insecticide (also known as phenthoate), was tested for its genotoxic effect on both maternal and fetal cells. Cidial was administered at three different dose levels (53.5, 106.9, and 171 mg/kg) to pregnant mice on day 16 of gestation. Maternal bone marrow and embryonic liver cells were examined for chromosomal aberrations and cellular proliferation. Cidial was found to increase the percentage of cells with chromosomal aberrations in both mothers and fetuses. It also significantly inhibited the rate of mitotic activity of both maternal and fetal cells, with the inhibitory effect being more appreciable in fetal cells than in maternal cells. The data indicate that cidial, which is widely used in rural areas, is hazardous to both mothers and their transplacentally exposed babies.