Gestational exposure to the fungicide dinocap produces behavioral abnormalities in the house mouse that are not apparent at birth but become obvious at weaning. Torticollis (head-tilting) appears in the treated offspring of CD-1 mice at three weeks of age and, during a test of swimming ability, many of the mice sink below the surface or are unstable and swim on their side in the water. These behavioral abnormalities are the result of agenesis of the otoliths in the inner ears. These behavioral and neurological alterations were the only developmental defects noted in the 12 mg/kg/d dosage group. In the group 4.4% of the mice displayed torticollis, 9.2% did not swim normally, 19% were missing one or more whole otoliths and partial agenesis of the crystalline material was seen in an additional 11.6% of the mice. The frequency of behavioral and inner ear defects increased in the higher dosage group, but the order of sensitivity of the effects did not change.