The closely related cyanobacteria Synechococcus and Prochlorococcus have different distributions in stratified water columns in the northern Sargasso Sea. The abundance of Synechococcus is relatively uniform with depth, but Prochlorococcus cell numbers are low within shallow mixed layers and high in and below the thermocline. Because free cupric ion (free Cu(2+)) concentrations are high (up to 6 pM) in shallow mixed layers and lower in deeper water, there is an inverse relationship between Prochlorococus its densities and the free Cu(2+) concentration. We explored the possibility of a causal underpinning for this relationship by examining the relative copper sensitivities of Prochlorococcus and Synechococcus in cultures and field Populations. Prochlorococcus isolates from both the high- and low-light adapted ecotypes were inhibited at free Cu(2+) concentrations that had no effect on Synechococcus. However, the high-light adapted strains were more copper resistant than their low-light adapted Counterparts. When copper was added to Prochlorococcus front environments where the in situ free Cu(2+) was low (in deeply mixed water columns and below the mixed layer in stratified conditions). net growth rates Were substantially reduced and cells arrested in the G(1) and early S phases of the cell cycle. Prochlorococcus in shallow mixed layers were less sensitive to copper and were probably members of the copper-resistant high-fight adapted ecotype. Synechoccus were relatively copper resistant across a range of environments in the Sargasso Sea. These observations are consistent with our hypothesis that copper plays a role in cyanobacteria ecology in the Sargasso Sea.