The acclimation response of the chloroplast was studied in the green marine microalga Koliella antarctica exposed to a simulated austral night of 90 d. On the basis of the micro- and submicroscopic aspects observed, the photosynthetic pigment patterns monitored spectrophotometrically, and the course of the assembly of the PSII chlorophyll-protein complexes evaluated microspectrofluorimetrically in vivo on single living cells, it was established that the alga tolerates the stress of light absence when cultured in the laboratory. During the treatment, the organism undergoes substantial structural and functional reorganization of the plastid, resulting in the formation of a chlorochromoplast-like structure, suitable for the storage, in different times and in specialized structures, of the products coming from the breakdown of the pre-existing plastid constituents. On the whole, the dark acclimation occurs in two different steps: a first acclimation, quickly realized, during the first 6 d and maintained up to 20 d of darkness, and a second acclimation period, starting from the 21st day, maintained up to the end of the experiment.