We have been able to isolate several phytotoxic compounds from aqueous extracts and leachates of cattails (Typha domingensis) using activated charcoal as an absorbant, followed by successive extraction with organic solvents, analysis by GC/MS, and structural elucidation by NMR spectroscopy when possible. The phytotoxins were identified as essential fatty acids (linoleic acid and alpha-linolenic acid) and phenolic compounds of known phytotoxic activity (caffeic acid from the aqueous extracts; caffeic, p-coumaric, and gallic acid from the leachates). Both extracts and the phytotoxins in the extracts have the potential of inhibiting the growth and chlorophyll production of several ecologically relevant species.