The chrysophyte genera Poterioochromonas and Ochromonas and their heterotrophic analogons, i.e. the 'Spumella-like' flagellates, account for a significant and often dominating fraction of the pelagic nanoplankton. Even though several osmotrophically and autotrophically grown strains of Ochromonas and Poterioochromonas are assumed to produce toxins, the potential toxicity has been investigated neither for its association with bacterivorous nutrition nor within the related exclusively heterotrophic 'Spumella-like' flagellates. We investigated the toxic potential of several flagellate strains using cultures of flagellates, cell extracts and filtrate of flagellate cultures. ne effect on potential predators was exemplarily tested for the cladoceran Daphnia magna and the rotifer Platyias sp. All tested heterotrophic and mixotrophic flagellate strains were toxic to zooplankton at abundances exceeding 10(4) flagellates mL(-1). For the rotfiers, survival on any of the flagellate strains was significantly lower than that in the control treatment (P < 0.001) already after 24 h. We conclude that (i) 'Spumella-like' flagellates can be toxic to Zooplankton, (ii) all tested flagellates, i.e. heterotrophic and mixotrophic flagellates feeding phagotrophically can be toxic to zooplankton and (iii) sublethal effects may be observed at typical field abundances, even though acute toxicity seems to be restricted to flagellate abundances observed only at peak events.