The metabolic organization of ketone body metabolism of liver and kidney of the goldfish Carassius auratus was assessed by measuring maximal activities, subcellular distribution, and stereoisomer preference of ketone body enzymes. These determinations indicate that the organization of ketone body metabolism in liver and kidney of goldfish differs from that of mammals in some respects. All the enzymes of ketone body metabolism were present in liver and kidney of goldfish, with the exception of hydroxymethylglutaryl-CoA (HMG-CoA) synthetase, which was not detected in liver. Two forms of Ξ²-hydroxybutyrate dehydrogenase (Ξ²HBDH) with different stereospecificity for Ξ²-hydroxybutyrate (D-and L-Ξ²-hydroxybutyrate) were detectable in liver and kidney. All of the ketone body enzymes measured in liver were mainly in the mitochondrial fraction, with the exception of D-and L-Ξ²HBDH, which were cytosolic. In kidney, HMG-CoA synthase, together with HMG-CoA lyase and acetoacetyl CoA thiolase (AcoAT), were found mainly in the mitochondrial fraction. L-Ξ²HBDH was mainly cytosolic in kidney, but by contrast with liver, D-Ξ²HBDH was mainly found in the mitochondria, and SKT was distributed in both the mitochondrial and cytosolic compartments.