Benomyl, a benzimidazole fungicide, produced ocular and craniocerebral malformations in fetal rats when administered to the dams by gavage in a dose of 62.4 mg/kg of maternal body weight/day on days 7-21 of gestation. Ocular anomalies included retinal dysplasia, cataracts, microphthalmia, and anophthalmia. These anomalies occurred in 43.3% of fetuses exposed to benomyl and a normal protein diet but increased to 62.5% when benomyl administration was combined with a protein deficient (8% casein) diet. Microscopic examination of the malformed eyes revealed that the most common abnormality, retinal dysplasia, consisted of rosettes of retinal cells and retinal infolding. The majority of rosettes had a single layer and a limiting membrane. Rosettes with two or three layers were also observed, particularly in fetuses exposed both to protein deficiency and benomyl. Although anophthalmia was identified macroscopically in five fetuses, only a single instance of true anophthalmia was found microscopically. These data support the results of previous investigators that benomyl induces ocular malformation and that protein deficiency enhances the teratogenic effects of benomyl. The disorderly proliferation of retinal cells and rosette formation resembled the periventricular cell masses that accumulate in brains exposed to benomyl and certain other teratogenic agents. The anti-tubulin action of benomyl is known to impair microtubule formation and it may produce brain and ocular malformations by disruption of neuronal proliferation and migration.