ABSTRACT: This study investigated the influence of light stress on predator-prey interactions between phytoplankton and heterotrophic dinoflagellates. Among the studied phytoplankton species (dinoflagellates, cryptophytes, and prymnesiophytes), the coccolithophore Emiliania huxleyi was particularly sensitive to light stress, as evidenced by photosynthetic efficiency (F_v/F_m) responses and flow cytometry measures of physiological condition. In addition, E. huxleyi was the only studied prey species that experienced reduced dinoflagellate predation when light stressed: predation on stressed E. huxleyi decreased by an average of 49% relative to controls (range: 23-80%). These light stress and predation responses applied to both calcifying and non-calcifying E. huxleyi strains. High light exposure led to elevated production of dissolved dimethylsulfoniopropionate (DMSP) in one strain of E. huxleyi, a signal that was associated with decreased predation. Direct effects of light stress on the 2 dinoflagellate predators also differed. Predation by Amphidinium longum was strongly decreased by UV exposure, while predation by Oxyrrhis marina was largely unaffected. Our findings highlight the deep diversity that is characteristic of the planktonic protists, and point to chemical signaling as a means by which the effects of light stress can propagate into predation interactions. When defining the ecological niche of planktonic protist species, stress responses should be considered along with the resource acquisition strategies that support growth.