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In search of a botanical algicide, 40 traditional medicinal plants were screened for antialgal activity against the bloom-forming cyanobacterium Microcystis aeruginosa using coexistence culture system assay. The results of the coexistence assay showed that significant inhibition of the algae at 800 mg L were observed for methanolic extracts of the root of Salvia miltiorrhiza (Radix Salviae Miltiorrhizae), rhizome of Acorus tatarinowii (Rhizoma Acori), rhizome of Polygonum cuspidatum (Rhizoma Polygoni), cortex of Phellodendron amurense (Cortex Phellodendri), and fruits of Crataegus pinnatifida (Fructus Crataegi). Methanol extract of these plants were further partitioned with petroleum ether, chloroform, ethyl acetate, and acetone to obtain allelopathically active fractions with various polarity. Among these fractions tested, the ethyl acetate extract of S. miltiorrhiza was observed to be more efficient than the other plant extracts with the inhibitory rate (IR) of 91.3% at 800 mg L and 50% effective concentrations (EC) values 98.9 mg L after 7 days, followed by chloroform extracts of A. tatarinowii, S. miltiorrhiza, and P. cuspidatum, and the petroleum ether extracts of A. tatarinowii with EC 102.5, 111.5, 122.9, and 130.0 mg L, respectively. [ABSTRACT FROM AUTHOR]