Synechococcus sp. PCC7942 was transformed with genes encoding poly-3-hydroxybutyrate(PHB)-synthesizing enzymes from Alcaligenes eutrophus by using a new plasmid vector. Under photoautotrophic and nitrogen-starved conditions, PHB content of the transformant was improved by CO2-enrichment in sparging. Supplement of acetate under nitrogen-starvation enhanced its PHB content by more than 25%. The mol wt and distribution of PHB from the cyanobacterial transformant were similar to those by Escherichia coli transformant with the same genes as described above.