Four cases are reported in each of which squamous epithelium was an incidental finding in surgically excised thyroid gland tissue. The occasional thyroid cyst lined throughout by squamous cells probably represents a persistent ultimo-branchial body, but the evidence indicates that the usual source of such cells in this gland is metaplasia of the follicular epithelium. An explanation is offered for the infrequency of this transformation in the thyroid, despite the frequent occurrence of the changes which predispose to epithelial metaplasia at other sites. There is no evidence to suggest that squamous cells arising in this gland by either of these means have any sinister significance.