Bioluminescent dinoflagellates grow at one third the rate of their competitors of equivalent size, such as diatoms . Despite this disadvantage, dinoflagellates successfully persist within phytoplankton communities and even form large blooms during favourable conditions. One explanation for this paradox is that bioluminescence acts as a defence that reduces losses to zooplankton grazers, such as copepods [2,3]. Lindström et al.  found that the dinoflagellate Lingulodinium polyedra (F.Stein) J.D.Dodge increase their bioluminescence in response to copepodamides , polar lipids exuded by copepod grazers, allowing for a brighter flash when bioluminescent capacity is stimulated. Here, we show that copepodamide-induced bioluminescence in L. polyedra causes a marked shift in the grazing preference of the copepod Acartia tonsa Dana. L. polyedra goes from being the preferred prey when non-bioluminescent to near complete rejection when pre-treated with copepodamides to induce a higher bioluminescent capacity. High-speed and low-light-sensitive videos show how L. polyedra cells flash upon contact with the copepod and are subsequently rejected, seemingly unharmed (Videos S1 and S2). Instead, A. tonsa shows compensatory feeding on the alternative prey. Prevett et al. show that copepod-induced bioluminescence in the dinoflagellate L. polyedra substantially reduces losses to copepod grazing. Copepods compensate by feeding on other, undefended phytoplankton, resulting in a competitive advantage for L. polyedra. High-speed and low-light sensitive films show copepod rejection of bioluminescent cells.