We examined adult representatives of marine echinoderms, molluscs, and crustaceans for the presence of fatty acid binding protein and enzymes of fatty acid catabolism. In all cases there was no evidence for an intracellular fatty acid binding protein or for enzymes involved in the mitochondrial import of long-chain fatty acids (carnitine palmitoyltransferase). In all cases, regardless of phyletic relationship, 3-L-hydroxyacyl CoA dehydrogenase activity was present indicating the ability to catabolize fatty acids in some manner. Two possibilities are offered in explanation. One scenario suggests that the absence of carnitine palmitoyltransferase and fatty acid binding protein and the presence of 3-hydroxyacyl CoA dehydrogenase result from a developmental change to adult stages from pre-adult stages where fatty acids, predominantly as triacylglycerides, are important for growth and buoyancy. Another possibility is an extra-mitochondrial β-oxidation to shorten long-chain fatty acids such as a peroxisomal system. In this light we report the detection of a peroxisomal oxidation of palmitoyl CoA, oleoyl CoA and octanoyl CoA in the ventricle and hepatopancreas of lobster (Homarus americanus) and hepatopancreas, but not the ventricle, of scallop (Placopecten magellanicus) as reflected in readily measured activities of acyl CoA oxidase. In addition, the tissues of the scallop and lobster contained significant activities of carnitine octanoyltransferase and carnitine acetyltransferase thought to be involved in the transmembrane transport of medium- and short-chain fatty acids. Chain-shortened fatty acids, because of greater solubility, may preclude the requirement of fatty acid binding protein.