The yield of red pepper has been decreasing due to continuous cropping in Korea. Although this decline had been mainly attributed to the occurrence of diseases, it was suggested that allelopathy may account for the decline. The growth inhibition of red pepper was assayed by a germination test using water or organic solvent extracts of leaf, stem, root and cultivates soil. In addition, the allelopathy of red pepper from other sources of competition was analyzed by using continuous root exudate trapping system. Methanol extracts of stem and root of red pepper strongly inhibited the radicle growth of red pepper. Methanol extracts of leaf and root and water extract of root inhibited the hypocotyl growth. The methanol and water extracts of red pepper root contained several phenolic acids such as p-hydroxybenzoic acid and vanillic acid. In the absence of an XAD-4 resin column, the growth of red pepper was inhibited by the root exudate as compared with the presence of the column. In addition, this exudate inhibited the germination of red pepper, and it showed a characteristic absorption peak as in the case of the methanol extract of cultivated soil at 254 nm. I was assumed that the red pepper decline under continuous cropping was due to allelopathy.