The mid portion of the lateral thyroid lobes from 64 fetuses was systematically analysed for the presence of ultimobranchial body nests. The nests showed a prevalence of 1/24 (4.2 per cent) or 13/40 (32.5 per cent) depending on whether a single- or a multi-step sectioning method was employed, respectively, and showed anatomical, morphological, and histochemical features similar to those of ultimobranchial postnatal thyroid solid cell nests. Histochemical studies revealed the presence of mucosubstances in 73 per cent of the cases, calcitonin-immunoreactive cells in 36 per cent, and both carcinoembryonic antigen and high-molecular-weight cytokeratin-immunoreactive epidermoid cells in 85.7 per cent, respectively. These findings indicate that these markers, which are also expressed by solid cell nests of the thyroid, are of value for the detection and tracing of ultimobranchial tissue to earlier stages of development. The findings of this study support the hypothesis that mucoepidermoid carcinomas of the thyroid are of ultimobranchial tissue origin.