The differential growth patterns of the bursa of Fabricius and thymus of Comet strain chicks were examined up to 60 days of age. It was found that the bursa increased in size, in relation to body weight, during the first 3 wk of life but thereafter underwent involution. The thymic tissue increased in weight throughout the period of investigation, from 3 to 58 days of age; when this total weight was related to the number of thymus glands, it was found that during the first days of life, thymus tissue was increased by the growth of pre-existing thymic glands but that this was followed by a phase when new thymus glands appeared with no increase in the size of the largest glands. Although the growth and involution patterns of the thymus and bursa differed it was found that, for each age group, a linear relationship existed between the bursa and thymus, suggesting that there may be a common control of development in the two organs.