Phytotoxic compounds obtained from methanol extracts of field-grown lantana (Lantana camara L.) were quantified in terms of their inhibition of ryegrass (Lolium multiflorum Lam.) seed germination and/or seedling growth. Subsequent partition of the aqueous fraction (derived from drying the MeOH extract in vacuo and redissolving in distilled water) at various pHs with solvents of differing polarity indicated the phytotoxic compounds were both polar and slightly acidic. Thin-layer Chromatographic separation of the acidic butanol fraction in a solvent mixture of butanol, acetic acid, water (4∶1∶5, by volume) yielded an active fraction with anR f value of 0.89-1.0 that was inhibitory to both root and shoot growth of ryegrass. Two other fractions withR f values of 0.04-0.23 and 0.41-0.57 were inhibitory only to root but not shoot growth.