ABSTRACT: Marine picophytoplankton are responsible for a significant proportion of primary production in the ocean, and they are most abundant in warm and oligotrophic oceans. We compiled 2 datasets to evaluate 2 hypotheses related to the direct effects of temperature on picophytoplankton growth: (1) the optimal growth temperatures of picophytoplankton are higher than those of other larger phytoplankton; and (2) the activation energies of picophytoplankton growth are higher than those of larger phytoplankton. We found that based on the laboratory data, the optimal temperatures for picophytoplankton growth were not significantly different from those of other phytoplankton after controlling the effect of environmental mean temperature; however, the activation energies of picophytoplankton were marginally significantly higher than those of larger phytoplankton. From the field data, the growth rates of Synechococcus and picoeukaryotes increased with increasing temperature and nitrate and chl a concentrations, whereas the growth rates of Prochlorococcus were not dependent on temperature and decreased with light intensity and nitrate concentrations. When keeping other factors constant, the activation energies of growth rates of Synechococcus and picoeukaryotes were 0.53 ± 0.07 (mean ± SE) and 0.62 ± 0.11 eV higher, respectively, than the 0.36 eV estimated for bulk phytoplankton. Our results suggest that Synechococcus and picoeukaryotes might benefit from warming in mesotrophic waters, and the growth of Prochlorococcus might be retarded by increasing temperature and light levels but might benefit from the increasing oligotrophication in oligotrophic surface oceans.