Sixteen strains of the red tide dinoflagellate Alexandrium spp. were tested for their short-term effects on the heterotrophic dinoflagellates Oblea rotunda and Oxyrrhis marina. Some Alexandrium strains, but not others, caused loss of motility and cell lysis of the heterotrophic dinoflagellates. A live counting procedure using O. marina was developed to quantify these toxic effects, which were compared with HPLC estimates of paralytic shellfish poisoning (PSP) toxin content. Within 5 strains, for which PSP toxins could be verified, both non-effective as well as effective strains were present and the same holds true for the other strains without detectable PSP toxins. This clearly indicates that the toxic effects are not due to PSP toxins. The observed effects are caused by extracellular substances, because O. marina did not ingest Alexandrium spp. and lytic effects are also found in cell-free culture medium. The immobilisation effect was strongly dependent on the Alexandrium spp. cell concentration. EC50 concentration (Alexandrium spp. cell concentration which caused 50 % immobilisation after 1 h exposure), as estimated for 5 effective strains, ranged from 2.1 · 103 cells ml-1 down to 0.6 ·103 cells ml-1. A quantitative comparison experiment showed that both heterotrophic dinoflagellate species are immobilised, with the thecate species (O. rotunda) being even more affected compared to the athecate O. marina.