In light-grown Spirodela oligorrhiza, a duckweed, chloroplasts complete differentiation in 2·5 days. When young green fronds are transferred to the dark, their plastids undergo ultrastructural changes that are retrogressive. Prolamellar bodies appear in the intergranal spaces within 12 to 18 h, while well-developed amyloplasts can be seen after 11 days. Mature green fronds maintained in the dark for up to 3 months remain unchanged. The results are discussed in the context of monotropic and reversible pathways of plastid differentiation in higher plants.