The discovery, geographic extent, and growth characteristics of a triploid, yellow-flowered Erythronium are reported. The morphology, ecology, and life cycle of the triploid are indistinguishable from Erythronium rostratum, a diploid species that bounds it on three sides. The triploid occurs at the western margin of the range of Erythronium rostratum, and is the predominant form across an eight-county region in northwestern Arkansas, northeastern Oklahoma, and southwestern Missouri. Populations of diploids and triploids share low frequencies of flowering individuals and aggressive vegetative reproduction. Triploids show reduced pollen fertility and seed production compared to the diploid. Identifiable mitotic chromosomes occur as pairs in the diploid and sets of three in the triploid. An autotriploid origin in southeastern Washington or southwestern Madison counties, Arkansas, is proposed.