The recent characterization of triallate-resistant lines of wild oat (Avena fatua L.) deÐcient in triallate sulfoxidation provides an experimental system to investigate and di †erentiate the e †ects of triallate and triallate sulfoxide on wax and lipid biosynthesis. Greenhouse applications of triallate dramatically reduced epicuticular wax deposition in susceptible (S) but not resistant (R) wild oats. Triallate treatment had no e †ect on in-vivo concentrations of C 12 to fatty acids and fatty alcohols in R plants, while elongated fatty acid frac-C 26 tions (C [ 18) were signiÐcantly reduced in S plants. In contrast, treatment with triallate sulfoxide reduced in-vivo concentrations of elongated fatty acids equally in R and S, supporting the hypothesis that triallate sulfoxide is more inhibitory than triallate towards fatty acid elongases. Although de-novo synthesis of shortchain fatty acids was not a †ected by triallate or triallate sulfoxide in R or S plants, synthesis of elongated fatty acid fractions was dramatically reduced in S plants by triallate. Fatty acid biosynthesis in R and S plants was equally sensitive to triallate sulfoxide. The results support the idea that in-vivo triallate sulfoxidation is necessary for herbicidal activity, and conÐrm that reduced rates of triallate sulfoxidation confer resistance in R wild oats.