Field surveys in summer and winter seasons showed that both the plant population and cover (%) of weed plants in fern-growing areas were lower than in areas without fern (Pteridium aquilinum L. Kuhn), showing the allelopathic effects of this fern. Hence, its allelopathic effects were studied. In bioassays, the water extracts of fern fronds and rhizomes, especially the fronds, showed persistent and higher allelopathic effects (inhibition of seed germination, hypocotyl and radicle elongation) in many vegetables, than water extracts of litter and soils around the fern plant. This phenomenon was observed throughout one year and may partly explain the long-term allelopathic effects of this fern In situ. The water-soluble allelochemicals significantly inhibited the mitosis of radicle tip cells in radish. These allelochemicals at low rates (6%, w/v), kept high proportion of cells under division in prophase, while at the highest rate (25%) tested, the cell division was completely maintained at prophase. The allelopathic effect of water extract of bracken fern was confirmed and an interruption in plant cell cycle might be involved in the mechanism of action.