Three strains of the marine dinoflagellate Alexandrium ostenfeldii of different geographic origin were tested for their short-term deleterious effects on a diversity of marine protists. All A. ostenfeldii strains were capable of eliciting an apparent allelochemical response, but the various protistan target species were differentially affected. Protists that were negatively affected by exposure to cells of A. ostenfeldii and associated extracellular metabolites comprised both autotrophs (Rhodomonas sp., Dunaliella salina, Thalassiosira weissflogii) and heterotrophs (Oxyrrhis marina, Amphidinium crassum, Rimostrombidium caudatum). Observed effects included immobilisation (e.g. of O. marina), morphological changes (e.g. in D. salina) and/or aberrant behaviour (e.g. of R. caudatum), mainly as preliminary stages of cell lysis. Immobilization and lytic effects against O. marina were strongly dependent on A. ostenfeldii cell concentrations. Effects also differed substantially among strains and different batch cultures of the same strain. Values of EC 50, defined as the A. ostenfeldii cell concentration causing lysis of 50% of O. marina cells, ranged from 0.3 to 1.9 × 103 mL -1, depending on the A. ostenfeldii strain. The autotrophic dinoflagellate Scrippsiella trochoidea reacted to exposure to A. ostenfeldii cells by formation of temporary (ecdysal) cysts, whereas, in contrast, the flagellates Emiliania huxleyi and Prymnesium parvum and the ciliate Strombidium sp. were relatively refractory or even unaffected. As long as cells did not lyse, the fluorescence yield of target autotrophs, estimated by pulse-amplitude modulation fluorometry, did not significantly change during the first 3 h of incubation, suggesting that allelochemicals produced by A. ostenfeldii caused no short-term negative effects on the photosynthetic apparatus. Overall, the allelochemical responses of target species showed no obvious relationship to cell quota or extracellular concentrations of either toxic macrocyclic imines (spirolides) or tetrahydropurine neurotoxins (saxitoxin and analogues) produced by various strains of A. ostenfeldii. Instead, the potency of A. ostenfeldii, eliciting immobilization and lytic species-specific responses in potential predators and competitors, is consistent with the existence of an allelochemical mechanism unrelated to the bioactivity of known phycotoxins of the genus Alexandrium. © The Author 2007. Published by Oxford University Press. All rights reserved.