Mesquite (Prosopis juliflora) is the leguminous shrubs growing in the tropical or subtropical regions. In this study, we conduct screening of biologically active substances produced by mesquite. Dried mesquite leaves (250g) were extracted with methanol. The extract (48.6g) was partitioned into the acidic and neutral fraction (22.2g) and the basic fraction (3.9g). The acidic and neutral fraction exhibited the inhibitory activity for root and shoot growth of lettuce seedlings. Furthermore, the basic fraction and the acidic and neutral fraction inhibited seed germination of Striga hermonthica, S. gesnerioides and Orobanche minor. The purification of these bioactive substances is in progress.