SUMMARY: 3-Hydroxybutyrate dehydrogenase was studied in 14 Rhizobium strains representing six species. Cell-free extracts of the bacteria were subjected to electrophoresis in starch or polyacrylamide gels and 3-hydroxybutyrate dehydrogenase was located in situ on the gels. Also, the rate of reaction of the enzyme, from different Rhizobium strains, with NAD was compared with its rate of reaction with various analogues of NAD. Most of the Rhizobium strains had characteristic 3-hydroxybutyrate dehydrogenase patterns on gel electropherograms and these patterns were highly reproducible. Multiple forms of 3-hydroxybutyrate dehydrogenase were detected in many of the Rhizobium strains.