<p>Harmful algal blooms (HABs) frequently occur and cause extensive damage to fisheries in coastal areas around the world. Minimizing the deleterious impacts of HABs requires a better understanding of the eco-physiological characteristics of HAB species and effective techniques for predicting dominant HAB species. In western coastal Japan, two HAB species, <i>Chattonella marina</i> complex (hereafter <i>Chattonella</i>) and <i>Karenia mikimotoi</i>, appear in summer, but one species typically dominates, suggesting that one species may potentially suppress the growth of the other. To elucidate the developmental mechanisms of mono-specific blooms of <i>Chattonella</i> and <i>K. mikimotoi</i>, we investigated growth interactions between the two species using bi-algal cultures. Bi-algal cultures with nine combinations of initial cell densities showed that <i>Chattonella</i> significantly inhibited the growth of <i>K. mikimotoi</i>. These interactions were quantitatively analyzed using the Lotka–Volterra competition equation, which showed that both species exhibited a competitive effect on the other, but <i>Chattonella</i> exerted a stronger effect on <i>K. mikimotoi</i>. However, culture filtrates containing extracellular substances of one species did not affect the growth of the other, and no growth inhibition was observed when the two species were cultured together under non-contact conditions. In addition, bi-algal cultures using one of the two <i>Chattonella</i> strains exhibiting different degrees of superoxide (·O₂⁻) production and <i>K. mikimotoi</i> demonstrated that, irrespective of ·O₂⁻ levels generated by <i>Chattonella</i>, the growth of <i>K. mikimotoi</i> declined similarly. These results indicate that direct contact with <i>Chattonella</i> cells, rather than extracellular allelopathic substances or ·O₂⁻ production, is central to cause the inhibition in <i>K. mikimotoi</i>.</p>