Nostocarboline (I) and seven derivatives were prepared and displayed minimal inhibitory concn. (MIC) values >=100 nM against the growth of Microcystis aeruginosa PCC 7806, Synechococcus PCC 6911, and Kirchneriella contorta SAG 11.81, probably via the inhibition of photosynthesis. The natural product hybrid nostocarboline/ciprofloxacin displayed addnl. antibacterial activity against several Gram-neg. bacteria (MICs >=0.7 micro M). Nostocarboline can thus be considered a potent, selective, readily available, natural algicide.