Peroxisomes are cytoplasmic organelles involved in fatty acid oxidation, ether-phospholipids biosynthesis, glyoxylate metabolism and purine catabolism among other metabolic pathways. However, these organelles are still poorly studied in molluscs. During a light and electron microscopy study of the digestive glands of cephalaspidean sea slugs unusually large peroxisomes reaching about 5 µm in diameter were found in basophilic cells of Philinopsis depicta (Aglajidae), being among the biggest ever reported in metazoan cells. These round or oval peroxisomes were clearly visible with the light microscope, and their diamond shaped core was strongly stained by the tetrazonium coupling reaction for protein detection. However, in the basophilic cells of Aglaja tricolorata, another aglajid cephalaspidean, peroxisomes were more variable in shape and no bigger than 1 µm in length. Round peroxisomes with a diameter of about 0.5 µm were common in basophilic cells of Philine quadripartita (Philinidae) and Haminoea navicula (Haminoidae), whereas in Bulla striata (Bullidae) these organelles frequently had a diameter of between 1.0 and 1.5 µm. Peroxisomes were larger in basophilic cells than in digestive ones. In all these species, the electron-dense peroxisomal cores were diamond-shaped, and in H. navicula two cores could be seen in each peroxisome. The abundance and large size of peroxisomes in the digestive gland points out the importance of these organelles in the metabolism of this organ.