The process of ossification in the cartilages of the larynx, trachea and syrinx of the domestic fowl has been studied in growing and in adult Golden Comet birds. In the laryngeal cartilages, mineralisation occurred consistently in the body and wings of the cricoid cartilage, in the procricoid cartilage and in the bodies of the arytenoid cartilages. In 7 out of 12 adult birds there were small additional centres in the tips of the rostral processes of the arytenoid cartilages and in one adult bird there were small centres in the caudal processes also. When present, these additional centres were always found bilaterally. Mineralisation in the laryngeal cartilages developed from 105 days post-hatching onwards and was first seen in the bodies of the arytenoid cartilages. Some evidence of a segmental pattern was noted, both in the earliest centres and in the final stage in the caudal region of the body of the tcricoid cartilage. Mineralisation in the trachea developed from 98 days post-hatching onwards. It was first found in the caudal region and spread craniad. The rings at the caudal end of the trachea and the cranial end of the tympanum (but not the last two rings of he tympanum) were always fully involved. Rings in the cranial part of the trachea remained more lightly mineralised. In the syrinx, mineralisation regularly occurred only in the pessulus and in the base of the first bronchial syringeal cartilages. In some birds, it was also encountered more caudally in the cartilages of the primary bronchi. Histological examination showed that, in the early stages, the alizarin staining was due to the presence of mineralised cartilage. At 182 days post-hatching and in the adults, actual bony tissue was observed. The possible significance of the occurrence of ossification in these cartilages of birds in briefly considered.