Ultrastructural studies of both young and harvest-ripe avocado fruits have established that the skin and outer green layers of flesh contain chloroplasts with an extensive thylakoid system. Etioplasts occur in the yellow flesh adjacent to the stone. The pale-green flesh contains plastids, intermediate between chloroplasts and etioplasts, which have prominent prolamellar bodies from which radiate grana. When segments of both the yellow and pale-green flesh of mature fruit (7 cm diam.) are cultured in the light their prolamellar bodies do not disperse although there is a change in their crystallinity. The palegreen tissues of immature (4 mm and 2 cm diam.) fruits also contain etioplasts but on culturing these differentiate into chloroplasts. Both chlorophyll content and the ratio of chlorophyll a to b varied in the different tissues of young and mature fruits.