Morphological and anatomical effects of mefenacet on the growth of barnyardgrass (Echinochloa oryzicola Vasing.) were investigated in petri-dish and pot tests. In the petri-dish, mefenacet did not inhibit weed germination. After germination, however, leaves failed to emerge through the coleoptile, and the primary root was reduced at high concentrations, 10 and 5ppm. At low concentrations, 0.5 and 0.1ppm, the growth of shoot, root and coleoptile was slightly inhibited, while the mesocotyl was not affected by the herbicide. In the preemergence treatment in pots, mefenacet did not inhibit the elongation of mesocotyl or coleoptile, however, it severely retarded the growth of 1st and 2nd leaf primordia and length of the primary root. When the weed was treated at 1, 2 and 3 leaf stages, the growth of primordia of subsequent leaves was retarded, and the elongation of coronal roots was inhibited. Anatomical results showed that mefenacet clearly exerted a strong inhibitory action against meristems, but had little effect on existing plant tissue. When applied at the 2-leaf stage, the growth of subsequent leaves was strongly retarded, as was the leaf sheath of the 3rd leaf. The upward and marginal growth of the 3rd and 4th leaves was inhibited and the growth of primordia of the 5th and 6th leaves was also severely inhibited. The growth of epidermal cell was affected and the intercellular spaces of the mesophyll did not form extensive lacunae as shown in the untreated plant. At the 3-leaf stage application, the growth of primordia of the 4th, 5th and 6th leaves was retarded and abnormal swelling of the 1st, 2nd and 3rd tillers was observed.