The activities of fatty acyl-CoA oxidase (FAO) and carnitine palmitoyl transferase (CPT), indices of the capacities of peroxisomal β-oxidation and mitochondrial β-oxidation, respectively, were determined in livers of several vertebrate species notable for differences in dietary fatty acid composition. In suckling rats FAO activities were half that in adult rats and CPT/FAO ratios twice that of adult rats. As their milk diet is dominated by medium chain fatty acids, this observation is consistent with current ideas about the role of peroxisomal β-oxidation in rat liver in oxidation of long chain unsaturated fatty acids. In nectar-feeding hummingbirds (fatty acids synthesized de novo) FAO activities were 50% greater than adult rats and CPT/FAO ratios onethird less than adult rats, suggesting that peroxisomal β-oxidation is relatively more important in this species, despite a fatty-acid-poor diet. In marine fish (herring, dogfish shark, hagfish) FAO activities were all less than 15% that of rats and undetectable in hagfish. CPT/FAO ratios were greater in herring (8.1) and hagfish (> 30) than adult rats (3.1), suggesting that peroxisomal β-oxidation is relatively less important in these species despite a natural diet containing high levels of long chain polyunsaturated fatty acids. These data are discussed in relation to current ideas about the role of peroxisomes in β-oxidation of fatty acids.