Occurrence and partial characterizati of lysozymes in the oyster Crassostrea gigas, and the mussel Mytilus edulis, were investigated. Lysozymes were fractionated on a chitin-coated cellulose column. The lysozyme activities were measured using both Micrococcus iysodeikticus and glycol chitin as substrates. In the oyster, high lysozyme activity was observed in the extracts of the digestive diverticula. Lower activities were detectable in the extracts of the gill, mantle, CrystallIne style, and in the hemolymph, though no activities were found in the adductor muscle. Similar results were obtained in the mussel. the oyster lysozyme activity was maximal at pH 5.5 and 50°C, whereas the mussel one was maximal at pH 5.5 and 60°C. The enzyme activities were inhibited mainly by Fe^g+ and iodine, whereas they were enhanced slightly by Ca²⁺ and Na⁺. Finally, a possible relationship between lysozyme and bactericidin was discussed.