A thermophilic cyanobacterium strain MA19 that photoautotrophically accumulates a high amount of poly-beta-hydroxybutyrate (PHB) was isolated and identified as Synechococcus sp. This strain accumulated PHB at more than 20% of the dry cell weight after the cells were cultivated in a nitrogen-free inorganic medium aerated with 2% CO2. The PHB content was the highest in all the photoautotrophic organisms. PHB was observed as intracellular granules like that previously reported for other bacteria and cyanobacteria. Supplementation of CO2 and light intensity were effective in increasing the rate of PHB accumulation by MA19. Addition of a nitrogen source resulted in reduction of PHB content. In nitrogen-depleted dark conditions, the PHB content increased to 27% because cell weight was reduced due to glycogen decomposition. MA19 is a novel candidate for PHB production from CO2 through a photosynthetic process.