In 14 collections of leafy spurge from Ontario and Saskatchewan the chromosome number is 2n = 60. The problem of the correct botanical name of this plant is reviewed; all the North American collections seen are classed as E. esula L. sensu lato. The morphology and cytology of an artificial hybrid, E. cyparissias × E. esula, is described; its chromosome number is 2n = 50 and meiosis is irregular. The name E. × figertii Dörfler usually applied to this hybrid is rejected in favor of the prior name E. × pseudo-esula Schur.