The coexistence of two thiolase isozymes (acetoacetyl-CoA thiolase and 3-ketoacyl-CoA thiolase), essential for the complete degradation of fatty acids, only in peroxisomes of an n-alkane-utilizing yeast Candida tropicalis (Kurihara et al., J. Biochem., 106, 474-478, 1989) is unique in eukaryotic cells. As one of the methods of analysis of molecular information from these isozymes, the calculation of the evolutional distance among thiolases from various organisms suggested that yeast peroxisomal thiolase isozymes are important enzymes in examining the molecular evolution of the fatty acid metabolic pathway and the biogenesis of peroxisomes.