We measured the carbon, nitrogen, and phosphorus content and production of cultured SAR11 cells in the genus Pelagibacter, from members of the 1a.1 and 1a.3 lineages, which are adapted to productive coastal waters and oligotrophic tropical/subtropical environments, respectively. The average growing SAR11 cell contained ~6.5 fg C, from which we calculated a global standing stock of 1.4 1013 g C. Calculations that consider uncertainties in cell turnover rates and growth efficiencies indicate this stock could oxidize 6 to 37% of gross ocean primary production. We also found that SAR11 do not incorporate 3H-thymidine, but do incorporate 3H-leucine. We estimate conversion factors of 0.74 - 1.51 kg C mol-1 leu, which are comparable to the low end of published leucine conversion factors for marine chemoheterotrophic bacterioplankton production. The molar ratio of elements C:N:P in growing cells was on average 25:6:1, significantly less than the mean (~50:10:1) for heterotrophic bacteria, indicating these strains are C and N poor relative to P.