Premetamorphic tadpoles of Xenopus laevis reared in water containing 0.01% propylthiouracil are developmentally retarded and metamorphosis is prevented. When uncrowded, they continue to grow to a giant size. Moderate crowding leads to a slower rate of growth. Thus morphologically premetamorphic tadpoles were produced with lens diameters appropriate to either normal premetamorphic, climactic or post-metamorphic animals. The lens crystallins of such tadpoles have been separated by immunoelectrophoresis and polyacrylamide gel electrophoresis. The crystallin pattern was correlated with lens diameter rather than with body stage. Giant retarded larvae possessed crystallin patterns identical to those or normal adults. The thyroid antagonist propylthiouracil is therefore unable to prevent the lens crystallin transition, the beginning of which is normally temporally correlated with metamorphosis.