A basal diet with practically no natural biotin was supplemented with 10, 19, 34, 62, 111 or 200 microgram of biotin per kg. Chicks receiving these graded levels of biotin were used for histological investigations of skin samples of the foot pad and the under and upper side of the middle toe. Where there was biotin deficiency, epidermal hyperplasia, papillary growth, acanthosis and hyperkeratosis were observed on the foot pad and the under side of the toe. In the stratum spinosum, cells showed big nuclei and nucleoi, and the glycogen-containing nuclear and perinuclear vacuoles had disappeared. The alterations in the skin of the upper side of the toe were similar but far less pronounced. Cell proliferation measured by the labelling index was significantly increased in all three skin samples.