Filtrates from crushed Moringa oleifera seeds were tested for their effects on growth and Photosystem II efficiency of the common bloom-forming cyanobacterium Microcystis aeruginosa. M. aeruginosa populations exhibited good growth in controls and treatments with 4- and 8-mg crushed Moringa seeds per liter, having similar growth rates of 0.50 (+/-0.01) per day. In exposures of 20- to 160-mg crushed Moringa seeds L(-1), growth rates were negative and on average -0.23 (+/-0.05) .day(-1). Presumably, in the higher doses of 20- to 160-mg crushed seeds per liter, the cyanobacteria died, which was supported by a rapid drop in the Photosystem II efficiency (Phi(PSII)), while the Phi(PSII) was high and unaffected in 0, 4, and 8 mg L(-1). High-density populations of M. aeruginosa (chlorophyll-a concentrations of approximately 270 microg L(-1)) were reduced to very low levels within 2 weeks of exposure to >/=80-mg crushed seeds per liter. At the highest dosage of 160 mg L(-1), the Phi(PSII) dropped to zero rapidly and remained nil during the course of the experiment (14 days). Hence, under laboratory conditions, a complete wipeout of the bloom could be achieved. This is the first study that yielded evidence for cyanobactericidal activity of filtrate from crushed Moringa seeds, suggesting that Moringa seed extracts might have a potential as an effect-oriented measure lessening cyanobacterial nuisance.