Production of (R)-3-hydroxybutyric acid [(R)-3-HB] by strain Halomonas sp. KM-1 under successive aeration conditions was investigated. The first aerobic condition allowed both cell growth and intracellular storage of poly-(R)-3-hydroxybutyric acid (PHB). The second microaerobic condition, achieved by reducing the culture agitation rate, lead to the degradation of PHB to (R)-3-HB. The amount of PHB stored in KM-1 cells after 48-h cultivation under aerobic conditions was 16.4 g/l. In contrast, after a shift from aerobic to microaerobic conditions and a further 18-h cultivation, PHB content in KM-1 cells decreased to 0.9 g/l. Numerous intracellular PHB-containing granules were observed in cells under aerobic conditions by electron microscopy. After the shift to microaerobic conditions, the number and size of granules were significantly reduced, in agreement with the degradation of prestored PHB. On the other hand, under microaerobic conditions, the concentration of (R)-3-HB in the medium reached a maximum of 15.2 g/l, indicating the production and extracellular secretion of (R)-3-HB as a result of PHB digestion. Notably, cell lysis was not observed during the successive aeration conditions as assessed by elution of genomic DNA to the culture supernatant, cell morphology observed by electron microscopy and counts of colony formation. In this simple system utilizing a change of aeration during cultivation of strain Halomonas sp. KM-1, we obtained one of the highest levels of microbiological production of (R)-3-HB reported to date.