Cyanobacteria are photoautotrophic organisms capable of oxygen-producing photosynthesis similar to that in eukaryotic algae and plants, and because of this, they have been used as model organisms for the study of the mechanism and regulation of oxygen-producing photosynthesis. To understand the entire genetic system in cyanobacteria, the nucleotide sequence of the entire genome of the unicellular cyanobacterium Synechocystis sp. PCC6803 has been determined. The total length of the circular genome is 3,573,470 bp, with a GC content of 47.7%. A total of 3,168 potential protein coding genes were assigned. Of these, 145 (4.6%) were identical to reported genes, and 1,259 (39.6%) and 342 (10.8%) showed similarity to reported and hypothetical genes, respectively. The remaining 1,422 (45.0%) showed no apparent similarity to any genes registered in the databases. Classification of the genes by their biological function and comparison of the gene complement with those of other organisms have revealed a variety of features of the genetic information characteristic of a photoautotrophic organism. The sequence data, as well as other information on the Synechocystis genome, is presented in CyanoBase on WWW [http:/(/)www.kazusa.or.jp/cyano/].