In a series of 21 human fetal thyroid glands examined histologically in serial sections, seven ultimobranchial body cysts were found. The position of these cysts correlated well with the distribution of calcitonin-containing cells found by previous investigators in the adult thyroid gland. Ultimobranchial body cysts found external to the thyroid lobes may offer a developmental explanation for the paucity of calcitonin found in some adult thyroid glands. The close developmental relationship between the parathyroid gland and the ultimobranchial body could explain the presence of calcitonin found in these glands in some adults.