The known compounds friedelin, maytensifolin B, ginkgetin, bilobetin, and amentoflavone as well as the new triterpene 3β-hydroxyfriedelan-16-one were isolated fromCelaenodendron mexicanum (Euphorbiaceae), an endemic tree of the Pacific Ocean coast of Mexico. The compounds' structures were established on the basis of spectral analysis. The biological effects of aqueous leachates, a CHCl3-MeOH extract and the isolated compounds of leaves and twigs were evaluated on the radicle growth of two plants,Amaranthus leucocarpus andEchinochloa crusgalli, and on the radial growth of three phytopathogenic fungi,Fusarium sp.,Helminthosporium sp., andAlternaria sp. The organic extracts of both leaves and twigs inhibitedAmaranthus and stimulatedEchinochloa radicle growth. On the contrary, friedelin and maytensifolin B stimulatedAmaranthus and inhibitedEchinochloa. The target fungi showed a different response to each treatment, from inhibition to stimulation.