Leafy spurge (Euphorbia spp.), an aggressive noxious perennial weed of North America, is a complex group that has been designated as several different species, including E. esula L. (# EPHES) and E. virgata Wald. & Kit. [E. waldsteinii (Sojak) Radcliffe-Smith]. Current classification keys are unsatisfactory in assigning plants in the field to specific taxa (or to a single taxon). These keys rely heavily upon morphological characteristics of leaves, but great variation in the leaves has been noted by us and previously reported by others. In this study we demonstrate qualitatively and quantitatively that the within-plant, within-clone, among-clone, and among-site variation in leaf morphology and triterpenoid content of the latex of leafy spurge is inherently high. Leaf characters were of little value in separating any of the accessions considered in our study. Latex triterpenoid profiles were useful in distinguishing E. lucida W. & K. × salicifolia Host. and E. salicifolia from European E. esula, E. waldsteinii, and E. sequieriana Neck. ssp. seguieriana, and all Montana accessions previously described from morphological studies. We concluded that Montana leafy spurge and the European E. esula, E. waldsteinii, and E. sequieriana belong to a single taxon: Euphorbia esula L.