Blooms of cyanobacteria are widespread in lakes and reservoirs throughout the world. Based on knowledge of aquatic microbial ecosystem1,2), we have surveyed the interactions between algae and algae-lysing bacteria3). In the course of the study, Sphingomonas sp. M-17 was found to produce a unique anti-cyanobacterial compound, argimicin A (1)4), which exhibited potent and selective activities against cyanobacteria5). The strain produced mainly argimicin A. Other minor anti-cyanobacterial constituents produced were not enough for structure determination. Modifications of culture conditions improved the amounts of argimicins B and C, the two minor components. This communication describes the isolation, the structural elucidation and the biological activities of argimicins B and C.