The unicellular thermoacidophilic red alga Galdieria sulphuraria is known to grow autotrophically as well as heterotrophically on a wide variety of organic carbon sources. Strain 107.79 becomes nearly colourless when grown heterotrophically on lactose. Under autotrophic conditions, a dominant chloroplast occupies most of the cell volume, while very small, yellow to colourless plastid-like organelles were observed in heterotrophic cells. Their exact number could not be established with certainty. To investigate the role of these algal proplastids, we developed a method to isolate intact organelles from heterotrophic cells via sucrose density-gradient centrifugation. Using electron microscopy, these organelles were shown to have a morphology similar to proplastids with a double membrane envelope, very few thylakoids and one to several dense bodies, apparently plastoglobuli. Investigation of the enzymatic capacities of isolated proplastids showed activity of triose phosphate isomerase and NAD-glyceraldehyde 3-phosphate dehydrogenase (GAPDH), but NADP-GAPDH, 3-phosphoglycerate kinase, fructose-1,6 P2 aldolase, fructose-1,6 P2 phosphatase, and malate dehydrogenase were not detected. These findings were supported by the pattern of plastid- and cytosol-specific isozymes of sugar phosphate metabolism and indicate that heterotrophically grown cells of G. sulphuraria 107.79 have not only lost their photosynthetic apparatus but also exhibit an extremely reduced enzyme complement in their dedifferentiated plastid organelle.