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Keywords Allelopathy ・ Growth and interaction ・ Heterosigma akashiwo ・ Prorocentrum minimum・Reactive oxygen species・Skeletonema costatumThis manuscript has not been submitted elsewhere in identical or similar form, nor will it be during the first three months after its submission to Hydrobiologia. 3Abstract We examined the growth and interactions between the bloom-forming flagellates Prorocentrum minimum and Heterosigma akashiwo using bi-algal culture experiments. When both species were inoculated at high cell densities, growth of H. akashiwo was inhibited by P. minimum. In other combinations of inoculation densities, the species first reaching the stationary phase substantially suppressed maximum cell densities of the other species, but the growth inhibition effect of P. minimum was stronger than that of H. akashiwo. We used a mathematical model to simulate growth and interactions of P. minimum and H. akashiwo in bi-algal cultures.The model indicated that P. minimum always out-competed H. akashiwo over time.Additional experiments showed that crude extracts from P. minimum and H. akashiwo cultures did not affect the growth of either species, but both strongly inhibited the growth of the bloom-forming diatom Skeletonema costatum. Further experiments showed that it was unlikely that reactive oxygen species produced by H. akashiwo were responsible for the inhibition of P. minimum growth.4