The ultimobranchial gland of the laying chicken consists of groups of C cells interspersed among a collection of intercommunicating follicles and ducts of variable size and shape. The epithelium lining this system ranges from squamous to columnar and includes stratified squamous and pseudostratified columnar elements. Four cell types are distinguished in this epithelium: F, mucous, C, and basal cells. F cells show microvilli and microfilaments. Pinocytotic activity and images of fusion of coated vesicles with the plasma membrane are evident. The rough-surfaced endoplasmic reticulum (RER) and the Golgi complex are moderately developed. Dense bodies are encountered apically in some cells. Mucous cells possess microvilli and secretory material in the typical form of partially fused droplets. C cells contain secretory granules and are invariably separated from the follicular lumen by other cell types. The smaller, pyramidal basal cells contain filaments, RER, small Golgi complexes, free ribosomes and hemidesmosomes. The lumina contain flocculent or granular material, cellular debris and desquamated cells. Morphological evidence demonstrates that features of the pharyngeal epithelium are retained and that the majority of the cell types, with the exception of C cells, are presumably nonendocrine.