<jats:p> Dinoflagellates are a major cause of harmful algal blooms (HABs), with consequences for coastal marine ecosystem functioning and services. <jats:italic>Alexandrium fundyense</jats:italic> (previously <jats:italic>Alexandrium tamarense</jats:italic> ) is one of the most abundant and widespread toxigenic species in the temperate Northern and Southern Hemisphere and produces paralytic shellfish poisoning toxins as well as lytic allelochemical substances. These bioactive compounds may support the success of <jats:italic>A. fundyense</jats:italic> and its ability to form blooms. Here we investigate the impact of grazing on monoclonal and mixed set-ups of highly (Alex2) and moderately (Alex4) allelochemically active <jats:italic>A. fundyense</jats:italic> strains and a non-allelochemically active conspecific (Alex5) by the heterotrophic dinoflagellate <jats:italic>Polykrikos kofoidii</jats:italic> . While Alex4 and particularly Alex5 were strongly grazed by <jats:italic>P. kofoidii</jats:italic> when offered alone, both strains grew well in the mixed assemblages (Alex4 + Alex5 and Alex2 + Alex5). Hence, the allelochemical active strains facilitated growth of the non-active strain by protecting the population as a whole against grazing. Based on our results, we argue that facilitation among clonal lineages within a species may partly explain the high genotypic and phenotypic diversity of <jats:italic>Alexandrium</jats:italic> populations. Populations of <jats:italic>Alexandrium</jats:italic> may comprise multiple cooperative traits that act in concert with intraspecific facilitation, and hence promote the success of this notorious HAB species. </jats:p>