ABSTRACT The thymic rudiment in mice homozygous for the recessive mutation nude (nu) has been compared to the normal at two stages: the 14-to 15-day foetus and the 1-to 2-day-old young. In the former, the normal thymus already comprises two adjacent lobes but in the nude it is represented by a pair of thin strands of tissue, each with a narrow central lumen. In further development these strands become thicker and vesiculated. They progress no further and are found in the 1-to 2-day young as small vestiges that are never populated with lymphoid cells. The thyroid, parathyroids and mandible of homozygous nude mice are normal, indicating that the complex of structures derived from the third branchial arch is not affected as a whole.