ABSTRACT: Noctiluca scintillans is a bloom-forming heterotrophic dinoflagellate that can ingest (and grow on) a number of phytoplankton prey, including several potentially toxic phytoplankton species. The current study documented (1) a range of N. scintillans growth rates (μ = -0.09 to 0.83 d^-1) on several species of harmful dinoflagellates and raphidophytes, including Heterosigma akashiwo and Akashiwo sanguinea, and (2) the first published growth rates on Lingulodinium polyedrum, Chattonella marina, and Alexandrium catenella. N. scintillans attained maximum growth rates (μ = 0.83 d^-1) on the raphidophyte H. akashiwo and negative growth rates (i.e. significant mortality) on the dinoflagellates A. catenella (μ = -0.03 d^-1) and A. sanguinea (μ = -0.08 d^-1) and the raphidophyte C. marina (μ = -0.09 d^-1). Toxin production by A. catenella did not appear to be responsible for negative effects on N. scintillans growth, as indicated by feeding experiments using mixed algal assemblages and the addition of high concentrations of purified dissolved saxitoxin (up to 16.73 ng ml^-1). However, growth of both N. scintillans and H. akashiwo was negatively affected when exposed to A. catenella culture and cell-free filtrate. These results suggest (1) a species-specific role of N. scintillans in top-down control of toxic bloom-forming dinoflagellates and raphidophytes, (2) direct, though not necessarily saxitoxin-dependent, inhibition of N. scintillans growth by A. catenella, and (3) indirect effects of A. catenella on N. scintillans growth through reduction in the availability of high-quality prey. Together, these results provide insights into the potentially significant role of N. scintillans as a grazer of blooms of these species.