Ethanolic extracts of the root hemiparasitic shrub Olax phyllunthi contain significant fractions of their amino N as S-ethenyl cysteine, a new naturally occurring compound. This amino acid was present in all plant parts, including seeds and seedlings which had not yet established haustorial connections with surrounding hosts. It was freely mobile in the xylem of 0. phyllu~thi but was not encountered in any of the 45 species of hosts which it was parasiti~ng in native habitat. The iden~ty of the compound was established from HPLC, GC-MS and NMR characteristics of the purified compound against authentic S-ethenyl-tcysteine.