The non-protein amino acid S-methyl cysteine (CH3-S-CH2 CH (NH2) COOH) was identified and assayed by HPLC and GC/MS analyses of ethanolic extracts of freshly-collected culms and rhizomes of Australian species of Restionaceae. Of 140 species examined, only 20 proved positive and two probably positive for the compound. Amounts in culms and rhizomes of these positive species varied from a trace to 74% of the ninhydrin-positive soluble amino N or from a trace to 33 mu mol per g fwt of tissue. Certain species showed substantial Variations in amounts of S-methyl cysteine between sites and seasons. Xylem bleeding sap was collected and analysed from 32 species. The nine species whose xylem sap was positive for S-methyl cysteine (0.5-23.5% of xylem total amino N) also rated positive in culms and rhizomes. Investment of N in S-methyl cysteine ranged from 0.1 to 6.3% of culm total N or 0.2-6.7% of rhizome total N. A possible role of the compound in protection against herbivory was examined. Positive species came from only three of the 19 currently recognized genera, Lepyrodia (16 positive, two possibly positive), Restio (one positive species) Loxocarya, (one positive species) and a further three as yet unnamed species. Under a proposed revision (B. Briggs and L. Johnson, pers. comm.), the positive species of Restio and the three unnamed species would be ascribed to a redefined genus (Loxocarya sens. orig.), thus restricting all species containing S-methyl cysteine to this new genus and Lepyrodia. The latter genus is changed under the proposed classification by transfer of certain species to the genus Sporadanthus. All six species proposed for such transfer lack the compound. Taxonomic implications of the findings are discussed.