Many bacterial genes and virulence factors are regulated by interbacterial and/or host–parasite chemical signals. We demonstrate that toxin production by a free‐living freshwater cyanobacterium is regulated in part by the presence of extracellular products of a eukaryotic green alga. In growth experiments, extracellular products made by the cyanobacterium Anabaena flos‐aquae contained both anatoxin and microcystin, and signifi...