Résumé : Abstract. The level of peroxidase activity utilizing ascorbic acid changes during the development of the green alga, Acetabularia mediterranea. During development almost parallel levels of peroxidase activity and ascorbic acid content are detectable: both steadily decrease as algae progress from very young, slowly growing cells to the rapid growth stage and then to cells exhibiting differentiation into primordium and cap. Changes in the levels of the enzyme and its substrate in the cytoplasm and periplasm were demonstrated using biochemical and cytochemical procedures. Concomitant with these developmental changes, we also observed changes in the stage‐specific patterns of ascorbic acid concentration: growing algae exhibit a pronounced negative apicobasal gradient of ascorbic acid. Acetabularia cultivated at 1,200 lux (the normal intensity in a 12‐h‐light/12‐h‐dark cycle) and at 700 lux (intensity at which growth is reduced, and cap formation is delayed) were also compared. The higher light intensity induced a moderate decrease in the ascorbic acid content without noticeable changes in the compartmental distribution in the cytoplasm and periplasm, and an increase in the level of periplasmic peroxidase activity with little change in the total peroxidase activity. Catalase was found to be present at very low levels and is unlikely to play a role in H2O2 catabolism. Possible roles for ascorbic acid and peroxidase in the development of Acetabularia are discussed. Copyright © 1987, Wiley Blackwell. All rights reserved