The delta 13C of suspended particulate organic matter (SPOM) in surface waters increased from -22.9 to -18.1% during April 25-May 31, 1989 at the JGOFS North Atlantic Bloom Experiment Site (NABE Site; 47 degrees N, 20 degrees W). During the same period, nearly parallel increases in sinking POM delta 13C were also found, although these values were usually lower than those of the corresponding SPOM. Consistent with the hypothesis that plankton delta 13C and [CO2 (aq)] are inversely related, the increases in both sinking and suspended POM delta 13C were highly negatively correlated with mixed-layer [CO2(aq)] that generally decreased from 13.2-10.1 micromoles/kg during the five weeks. The change in SPOM delta 13C per change in [CO2(aq)], however, appears to be somewhat greater than that expected from previous, though less direct, ocean and laboratory evidence. By adapting a model of plant delta 13C by FARQUHAR et al. (1982), it is shown that under a constant phytoplankton demand for CO2 an inverse, nonlinear SPOM delta 13C response to ambient [CO2(aq)] is expected. Such trends are unlike the negative linear relationships indicated by data from the NABE Site and or from Southern Hemisphere waters. Such differences between predicted and observed SPOM delta 13C vs. [CO2(aq)] trends and among observed relationships can be reconciled, however, if biological CO2 demand is allowed to vary. This has significant implications for the use of the delta 13C of plankton (or their organic subfractions or sedimentary remains) as a proxy for past or present ocean CO2 concentrations and biological productivity.