Five mutant strains of Hydrogenomonas H 16 which synthesize poly-β-hydroxybutyric acid either slowly or not at all have been isolated following nitrite and NMG treatment of wild type cells. When grown on a nitrogen deficient agar medium, the colonies of PHB-free cells can be recognized by their diminished retention of the dye sudanblack B. Enrichment procedures for such mutants have been devised employing the 32P-phosphate inactivation technique and sucrose gradient centrifugation. The mutants have been characterized with respect to their growth properties, respiratory control and other properties.