Phytoplankton size structure is acknowledged as a fundamental property determining energy flow through 'microbial' or 'herbivore' pathways(1). The balance between these two pathways determines the ability of the ecosystem to recycle carbon within the upper layer or to export it to the ocean interior(1). Small cells are usually characteristic of oligotrophic, stratified ocean waters, in which regenerated ammonium is the only available form of inorganic nitrogen and recycling dominates. Large cells seem to characterize phytoplankton in which inputs of nitrate enter the euphotic layer and exported production is higher(2-4). But the size structure of phytoplankton may depend more directly on hydrodynamical forces than on the source of available nitrogen(5-7). Here we present an empirical model that relates the magnitude of mesoscale vertical motion to the slope of the size-abundance spectrum(8-10) of phytoplankton in a frontal ecosystem. Our model indicates that the relative proportion of large cells increases with the magnitude of the upward velocity. This suggests that mesoscale vertical motion-a ubiquitous feature of eddies and unstable fronts-controls directly the size structure of phytoplankton in the ocean.