SUMMARY The allelopathic influence of Parthenium hysterphorus, a gregarious and ubiquitous weed, on four multi-purpose tree species – Acacia leucophloea, Casuarina equisetifolia, Eucalyptus tereticornis and Leucaena leucocephala and three agricultural crops – cowpea, sorghum and sunflower – were tested by germinating seeds of these species in composite leachate obtained from the leaves, stem and flowers of the weed species. Germination of seeds of all test species, arboreal and arable, was severely inhibited by the leachate. In the tree crops, the magnitude of injury was the same under extracts prepared from 2 or 4 ml of distilled water per g of plant tissue. Among the arable crops, sunflower was affected to a greater extent. While radicle and plumule elongation were unaffected in the tree crops and in sunflower, plumule was curbed to a greater extent than radicle in cowpea. In sorghum, only radicle growth was impaired. The inhibition of germination and seedling growth is attributed to parthenin, an unsaturated lactone found in the plant parts of the weed species.