Allelopathic activity of 4 different types of weeds-Japanese native weeds (38 species of 18 families), naturalized or established weeds in Japan (30 species of 12 families), newly imported weeds (18 species of 8 families, originating as feed-mix in imported feeds), and Brazilian weeds (55 species of 17 families, considered potentially invasive), and total 129 species of 25 families were evaluated by "Plant-Box" method using lettuce as a test plant.<br>Results showed that the imported weeds had not always lower allelopathic activity than that of naturalized or established ones. However, several Brazilian weeds showed high allelopathic activities in comparison to native, naturalized or established weeds. The velvetleaf (either the imported or endemic type) showed strong phytotoxic effect against lettuce radicle growth. In case of common lambsquarters, strong allelopathic activities were observed on the imported type and that with Brazilian origin, but allelopathic activity of those phenotypes that already established in Japan was not prominent. These results suggest that allelopathic activity might depend on their origin of each phenotype. This study suggests that some potentially invasive weeds, as well as some imported weeds, were highly allelopathic, and could possibly become a threat to Japanese vegetation, if they invade, and establish in Japan.