A phytochemical study of the methanol-soluble fraction of an aqueous extract of a sample of Psittacanthus calyculatus collected from the host plant Prosopsis laevigata (Smooth Mesquite) using several techniques, including co-chromatography coupled with UV detection, chromatographic purifications and IR, NMR and MS studies, resulted in the identification of gallic acid, two flavonol-3-biosides and the nonprotein amino acid N-methyl-trans-4-hydroxy-L-proline.