1. Anaerobic energy metabolism was investigated in different organs of Mytilus edulis and the whole animal.2. Succinate accumulates to high levels in most organs but remains low in the hemolymph. 3. After 16 hours propionate accumulation is observed in all organs. Experimental evidence is not sufficient yet to point out organs that produce more propionate than others.4. Acetate is a minor end product. 5. Acetate and propionate are found in the hemolymph in amounts equal to those in the organs. 6. Animals incubated in oxygen-free seawater accumulate more end products than animals exposed to air, in the form of volatile fatty acids that are excreted into the incubation water.7. Alanine and glutamate increase in the posterior adductor muscle. Aspartate decreases in the total animal, posterior adductor muscle and gills, while in the hemolymph decreases in alanine, asparagine, serine, threonine and proline are observed.