The aim of the study was to measure toxin production and growth of the cyanobacterium Anabaena sp. in the presence of competitor algae and grazers. The study was comparative, as it was repeated at two sites in the Baltic Sea. The results showed that growth and intracellular microcystin concentrations of Anabaena were significantly higher at the Bothnian Sea site than at the Gulf of Finland. Toxin concentrations of Anabaena were higher in the presence of chlorophyte Brachiomonas submarina than in incubations with cryptophyte Rhodomonas salina or copepod Eurytemora affinis. Chlorophyll content was higher in the presence of copepods. The results suggest that both growth and toxin production of Anabaena may be controlled by salinity, because salinity is lower at the Bothnian Sea site, whereas also other factors could have influenced the results. Our data are also in line with the hypothesis that possible allelopathic interactions (here measured as toxicity) are more probable in the presence of foreign species. (C) 2011 Elsevier B.V. All rights reserved.