We studied allelopathy in the dinoflagellate genus Alexandrium by testing the effect of A. tamarense on a natural plankton community from Hopavagen Bay, Trondheimsfjord, Norway, and the effect of toxic and non-toxic strains of A. tamarense and a toxic strain of A. minutum on algal monocultures. Also, a possible relation between the allelopathic effect and the production of paralytic shellfish poison (PSP) toxin was investigated. A. tamarense affected the whole phytoplankton community by decreasing the growth rate and changing the community structure (relative abundance of each species, dominant species). A negative effect of A. tamarense was also observed on ciliates, but not on bacteria numbers, In the bioassay with algal monocultures, the diatom Thalassiosira weissflogii and the cryptophyte Rhodomonas sp. were exposed to the filtrate of Alexandrium spp. All tested Alexandrium strains negatively affected T weissflogii and Rhodomonas sp. cultures, independent of whether PSP toxins were produced. The compounds released by Alexandrium caused lysis of natural and cultured algal cells, suggesting that the allelopathic effect may be connected with previously described ichthyotoxic and haemolytic properties of Alexandrium. Furthermore, the observation that several components of the plankton community were affected by compounds released by A. tamarense emphasizes the importance of allelopathy for the ecology of this species.