Barnacle embryos were reared in millipore cytology monitors containing approximate tritiated water (HTO) concentrations of background plus 0, 10−5, 10−4, 10−3, 10−2, 10−1, and 100 microci/ml. After 32 days the cultures were fixed and the numbers of larvae counted. A “molting index,” the percentage of larvae that molted at least once, was used to evaluate the effects of HTO on normal development. Effects were observed at concentrations as low as 7 × 10−6 microci/ml, and were exponentially related to HTO concentration. Factors affecting sensitivity and possible environmental implications are discussed.