A sheathed bacterium Sphaerotilus natans could not survive at 4 degrees C for 2 months, and mutants that exhibited different colony phenotypes were obtained only by repeating the short : period of storage at 4 degrees C. The ability of these mutants and the parent strain to produce poly-3-hydroxybutyrate (PHB) was compared in batch cultures, The parent strain accumulated 30% (w/w) PHB, while one of the mutants defective in sheath formation, designated as T2, accumulated over 50% PHB. Because T2 did not require strict air or nitrogen limitation for polymer accumulation, its production was growth-associated, allowing one-stage fermentation, In a pH-controlled fermentation using a jar fermenter, 10 g/l glucose was converted into 2.0 g/l PHB in 24 h.