Abstract: Effect of Argemone mexicana L. on seed germination and early seedling growth of six crop species was investigated. Aqueous extract of A. mexicana inhibited germination, root and shoot growth of all the test species. The species showed differential response. Germination was reduced by the aqueous shoot extract in the order: pearl-millet > mustard > wheat> carrot> corn > turnip. Root length was reduced in the order:corn> pearl-millet> wheat > mustard > turnip> carrot, whereas shoot length was reduced in the order: pearl-millet> wheat> corn > turnip> mustard > carrot. Decaying A. mexicana in sandy loam soil at a rate of 5,10,20 g / 400 g soil substantialy inhibited germination and seedling growth of pearl-millet. Bioassay of the extract of A. mexicana revealed three phenolic inhibitors and paper chromatography revealed the presence of p-hydroxybenzoic, vanillic and salicylic acids.