Bromofenofos, an organophosphorus anthelmintic, was suspended in deionized water and administered once daily to pregnant rats by gastric intubation on days 8 through 15 of pregnancy in doses of 0, 2.5, 5, 10 and 20 mg/kg. The dams were killed on day 21 of pregnancy, and the number of implants, resorptions and live fetuses was counted. All fetuses were weighed and examined by routine teratological methods. As the results showed, this compound was highly embryolethal in the 20 mg/kg group; approx. 91% of the implants were resorbed at this dose level. Although none of the fetuses were externally malformed in any group, the incidence of skeletal and internal malformations was significantly increased in fetuses in the 20 mg/kg group. Skeletal malformations observed at this dose level were bipartite vertebral centra and wavy ribs. Internal malformations involved anophthalmia, hydronephrosis and hypoplasia of the uterus.