The contribution of bicarbonate to total dissolved inorganic carbon (DIC) utilization was investigated using 18 marine phytoplankton species, including members of Bacillariophyceae, Dinophyceae, Prymnesiophyceae, and Raphidophyceae, under carbon-replete or -limited conditions. Extracellular carbonic anhydrase (CA) was assayed as an indicator of extracellular CA-catalyzed HCO−3 utilization. For some species, extracellular CA was constitutive, in others activity was detected under conditions of carbon limitation, and in others, even under carbon-limited conditions, activity was not detected. In species without extracellular CA, direct HCO−3 uptake was investigated using a pH drift technique in a closed system, DIC measurements, and the use of the anion exchange inhibitor 4′4′-diisothiocyanatostilbene-2,2-disulfonic acid (DLDS). Three of these species (Chaetoceros compressus, Thalassiosira pseudonana, and Glenodinium foliaceum) gave a pH drift not inhibited by DIDS, but cultures of Chrysochromulina kappa, Gephrocapsa oceanica,and Coccolithus pelagicus, in which DLDS inhibited DIC uptake, did not give a pH drift. This result shows that direct HCO3− transport may occur by an anion exchange-type mechanism in some species but not others. Of the eighteen species investigated, only Heterosigma akashiwo did not have the potential for direct uptake or extracellular CA-catalyzed HCO−3 utilization.