Thyroid lobes of the newborn opossum consist of small epithelial cords or plates, the constitutent cells of which are joined by tight junctions and surround a minute lumen that courses throughout the length of each cord. Occasional cells, not directly associated with the lumina of the cords, show central nuclei and clusters of small electron-dense granules: these may represent parafollicular cells at an early state of development. By the end of the 1st week after birth, parafollicular cells are well established and appear as groups between the epithelial cords and as scattered individual cells within the cords. Growth of the thyroid occurs as the cords continue to elongate and branch. 11 days after birth, thyroid development in the opossum corresponds morphologically to that of eutherians just prior to birth. Primary follicles are present near the periphery of the thyroid lobes, having formed from evaginations of the epithelial cords. There is evidence of synthetic activity by the epithelial cells and colloid is present. Secondary follicles form as a result of evaginations from the epithelium of the primary follicles. At the end of the 2nd postnatal week, epithelial cords are no longer a prominent feature of the opossum thyroid, and the organ is now characterized by numerous, small colloid-filled follicles. As development progresses, the follicles increase in size and number and, by 35 days after birth, the thyroid shows adult features.