Poly-beta-hydroxybutyrate (PHB) from various representative strains of the genera Azotobacter, Beijerinckia, and Derxia was isolated and characterized. During growth in shake culture, with glucose as a carbon and energy source, and molecular nitrogen as a nitrogen source, increase in dry weight appeared linear, and PHB formed a constant percentage of the dry weight. In a medium containing 1% (w/v) glucose, PHB declined with the onset of the stationary phase of growth; with 2% (w/v) glucose, an increase in PHB content during stationary phase was noted in the case of some strains, before a subsequent decline. The decrease in PHB as a percentage of dry cellular weight (not of total amount present in the culture) during growth of some strains with 2% as opposed to 1% (w/v) glucose may be ascribed to a greater production of capsular polysaccharide. PHB content could not be used as a taxonomic criterion. Strain differences were as great as or greater than species differences. The only strain of Beijerinckia fluminensis obtained contained PHB, but it could not be grown on the nitrogen-free medium used. Two species of the genus Azotomonas, reported to be aerobic, nonsymbiotic nitrogen-fixers, did not grow on the nitrogen-free medium used and did not produce PHB during growth with a combined nitrogen source.