The granules observed in the cytoplasm of cells of sporogenic and asporogenic strains of Clostridium botulinum type E were isolated at various developmental stages of growth and sporulation. Electron microscopy of thin sections showed that most of the granules were dispersed throughout the cytoplasm. Chemical analysis and electron microscopy showed that the granules were poly-beta-hydroxybutyrate (PHB). The polymer began to accumulate after 8 h of growth, reaching 9 and 13% of the cell dry weight in the sporogenic and asporogenic strains, respectively, during early stationary phase. (14)C-acetate was readily incorporated into PHB. The rate of assimilation paralleled the production and utilization of PHB, indicating that the acetate served as its precursor. (14)C-butyric acid was not utilized to any significant extent. Most of the PHB which had accumulated in the sporogenic strain was catabolized during the development of the spore, but in the asporogenic mutant it remained essentially unchanged. The findings suggest that the PHB provides endogenous carbon and energy for the synthesis of spore-specific components required for spore maturation.