Marine cyanobacterium Synechococcus sp. strain. PCC 7002 has at least six endogenous plasmids. When it was cultured in 1 M NaCl medium, the copy numbers of the smallest (4.5 kb) and the second smallest (9.7 kb) plasmids decreased to one-third and one-tenth of those in control culture (0.3 M NaCl) respectively. In medium without NaCl, the copy numbers of those plasmids also decreased but the changes were much smaller. On the other hand, copy numbers of 15.4-kb, 30.0-kb, and 36.9-kb plasmids did not change among the three different NaCl concentrations (0 M, 0.3 M, 1 M). The copy number changes of the two plasmids were reversible. A similar copy number change was also observed in medium with 0.3 M NaCl + 0.7 M KCl. These observations suggest that copy number controls are different among endogenous plasmids, and some of them are affected by salinity in the medium.