Radioactive cysteine fed for 6 h to young gametophytes of Ceratopteris actively producing antheridia was incorporated readily into the early stages of spermatogenesis, but hardly at all into differentiating spermatocytes. Spermatocytes contained only little radioactivity even when the gametophytes were grown for a further 18 h before fixation, thus ensuring that the radioactive label was available at or near the time of their formation. There was some evidence that in very young spermatocytes in the latter material the incorporation was more marked at the periphery of the blepharoplast than elsewhere. The results are interpreted as indicating that the differentiation of the spermatocyte involves little if any net protein synthesis. The proteins of the new structures peculiar to the gamete may thus have been synthesized in earlier stages of spermatogenesis, the last phase of visual differentiation being a consequence of their assembly and organization.