Phosphamidon, an organophosphate pesticide, is an established cholinesterase inhibitor. Alteration of tissue and plasma cholinesterase activity at a critical developmental period may influence cellular division and growth sufficiently to produce anatomically or functionally abnormal tissue or organ. The present study was, therefore, undertaken to evaluate the teratogenic potential of phosphamidon in pregnant Swiss albino mice, when administered at different gestational days during the period of organogenesis. The animals were sacrificed on day 18 of gestation for routine teratological examinations. It was observed that phosphamidon was more embryotoxic than teratogenic. Maximum effects were observed when administered on day 7 and day 13. Treatment on day 10 produced little effects. Repeated exposure during the organogenetic phase also produced significant adverse effects. This possibly indicates that phosphamidon is more embryotoxic during the post-implantation period (day 7) and during late organogenesis (day 13) as compared to the early organogenesis period (day 10).