The genome of the marine cyanobacterium Prochlorococcus marinus SS120 consists of a single circular chromosome of 1.75 Mb; its mean G+C content is 36.4%. This low GC% was an advantage during the cloning phase: the SS120 strain cannot be cultivated axenically, i.e. grows in the presence of an contaminating organism with a much higher GC%, around 60%. Therefore, cloning conditions and insert sizes were chosen to minimize the proportion of high GC% clones. Contamination was lowest for inserts of 7 and 10 kb. Two libraries were thus constructed with genomic fragments of this size in the low copy vector pCNS, and 38,765 reads were produced at both ends. Around 23,500 reads have been used in the assembly, which translates in a depth of 8X. The gaps and regions of poor quality in the assembled sequence were subjected to finishing. The integrity of the finished sequence has been verified by comparison of theoretical and experimental restriction profiles of two enzymes. The annotation was performed by the members of the consortium (see main page).