The Galician Rías (NW Iberian Peninsula) are an important shellfish aquaculture area periodically affected by toxic episodes often caused by dinoflagellates such asDinophysis acuminataandAlexandrium minutum, among others. In turn, water discolorations are mostly associated with non-toxic organisms such as the heterotrophic dinoflagellateNoctiluca scintillans, a voracious non-selective predator. The objective of this work was to study the biological interactions among these dinoflagellates and their outcome in terms of survival, growth and toxins content. To that aim, short experiments (4 days) were carried out on mixed cultures withN. scintillans(20 cells mL−1) and (i) one strain ofD. acuminata(50, 100 and 500 cells mL−1) and (ii) two strains ofA. minutum(100, 500 and 1000 cells mL−1). Cultures ofN. scintillanswith twoA. minutumcollapsed by the end of the assays. BothD. acuminataandA. minutumexposed toN. scintillansarrested its growth, though feeding vacuoles in the latter rarely contained any prey. Toxin analyses at the end of the experiment showed an increase in intracellular OA levels inD. acuminataand a significant reduction in PSTs in bothA. minutumstrains. Neither OA nor PSTs were detected inN. scintillans. Overall, the present study indicated that the interactions among them were ruled by negative allelopathic effects.