||There are many unusual features of energy metabolism in African trypanosomes, the unicellular eukaryotic parasites responsible for African Sleeping Sickness. For example, several of the glycolytic enzymes are compartmentalized and are regulated uniquely. I have identified an unusual trypanosomal β-hydroxybutyrate dehydrogenase (HBDH), an enzyme involved in the production of ketone bodies. This enzyme, which catalyzes the reversible NADH-dependent conversion of acetoacetate to hydroxybutyrate, closely resembles bacterial forms of the enzyme. Furthermore, unlike many of its homologs in higher eukaryotes, the trypanosome enzyme appears not to be a membrane protein. This enzyme has been cloned from Trypanosoma brucei genomic DNA, soluble protein has been overexpressed in E. coli, and active enzyme has been purified to approximate homogeneity. I have identified this enzyme as having β-hydroxybutyrate dehydrogenase activity. Potential substrates have been tested and kinetic studies have been performed to determine K m values for these substrates of the trypanosomal β-hydroxybutyrate dehydrogenase.