​We determined the rates of photosynthesis and respiration in batch cultures of 22 marine phytoplankton species from five phyla covering a range of 7 orders of magnitude in cell size. Rates were determined during the exponential growth phase and also during the stationary phase, when cell growth was limited by nitrogen availability. We observed, in all growth phases, a curvature in the size scaling of carbon fixation, such that the relationship between carbon-specific photosynthesis and cell size was unimodal, with the highest rates being measured in intermediate-size species. The log–log relationship between individual metabolic rates and cell size showed an overall lineal pattern with a slope equal or near 1, irrespective of whether volume or carbon is used as a metric for cell size. Thus, our results demonstrate that when small species (< 50 μm3 cell diameter) are considered together with intermediate- and large-sized species phytoplankton metabolism does not follow Kleiber's 3/4-power rule. Considering all species together, respiration losses represented on average 9% and 22% of total carbon fixation during the exponential growth and stationary phases, respectively. Carbon-specific respiration was largely independent of cell size and growth phase, but tended to take higher values in the dinoflagellates. During the stationary growth phase, and contrary to other groups, most diatoms were able to maintain carbon fixation rates similar to those measured during exponential growth. Our results highlight the ability of intermediate-to-large size species to sustain high metabolic rates in spite of their cell size, which helps to explain why they dominate phytoplankton blooms in the ocean.