The effect of 2 strains (Alex2 and Alex5) of the marine red tide dinoflagellate Alexandrium tamarense on 10 other planktonic algal target species common in temperate waters was studied in mixed growth experiments under nutrient-rich conditions. In a comparative approach, the 2 strains of A. tamarense, similar in their cellular paralytic shellfish toxin (PST) content, were selected because of their fundamentally different lytic potencies. The Alex2 strain clearly affected all target algae while the Alex5 strain had no negative effect on the growth of any of the target species during the study period, even though cell concentrations of Alex5 became very high (2 × 104 cells ml-1). As both strains contained comparable amounts of PST, this confirmed previous suggestions that so far unidentified compounds are causing the negative effects on other algae. Sensitivity of the tested algae to Alex2 differed considerably. The growth of some species was affected at very low Alex2 cell concentrations (<102 cells ml -1), while the growth of other algae was not affected until cell concentrations exceeded 103 cells ml-1. While a complete dieoff was the ultimate fate for almost all target species when grown in mixed culture with Alex2, Scrippsiella trochoidea formed temporary cysts, the number of which rconstant during the course of the experiment. The pH in the mixed cultures increased as the cultures grew dense. This had a substantial effect on Alex5 in the mixed cultures, in which Alex5 eventually died off because the target species have a higher tolerance to high pH. pH values did not determine the outcome of the experiments with Alex2 because the adverse effects of Alex2 on the growth of the other algae was evident before pH values became too high. Lytic extracellular compounds, which are produced by the large majority of A. tamarense strains tested so far, clearly have the potential to benefit this dinoflagellate by reducing competitor growth rates. © Inter-Research 2009.