Allelopathy, the release of extracellular compounds that inhibit the growth of other microorganisms, may be one factor contributing to the formation and/or maintenance of cyanobacterial blooms. We investigated the allelopathic effects of three cyanobacterial species ( Nodularia spumigena, Aphanizomenon flos-aquae and Anabaena lemmermannii) that frequently form mass-occurrences in the Baltic Sea. We exposed monocultures of three phytoplankton species ( Thalassiosira weissflogii, Rhodomonas sp. and Prymnesium parvum) to cell-free filtrates of the three cyanobacteria, and quantified allelopathic effects with cell counts. We also investigated the role of the growth phase of cyanobacteria in their allelopathy, by comparing the effects of an exponential and a stationary phase culture of N. spumigena. All tested cyanobacteria inhibited the growth of Rhodomonas sp., but none of them affected P. parvum. The effects on T. weissflogii were more variable, and they were amplified by repeated filtrate additions compared to a single filtrate addition. N. spumigena was more allelopathic in exponential than in stationary growth phase, whereas the culture filtrate was more hepatotoxic in stationary phase. Hepatotoxins were thus probably not involved in the allelopathic effects, which is also indicated by the allelopathic properties of the non-toxic A. flos-aquae and A. lemmermannii. The results demonstrate that the common Baltic cyanobacteria affect some coexisting phytoplankton species negatively. Allelopathy may therefore play a role in interspecific competition and contribute to cyanobacterial bloom maintenance.