In a study of ten taxa ofChenopodium occurring primarily in the western United States and designated as the narrow-leaved complex, data from morphology, flavonoid chemistry, and somatic chromosome karyotypes were correlated. All taxa produce 3-O-glycosides of quercetin and/or isorhamnetin. The six taxa examined for chromosome number were found to be diploid (2n = 18).Chenopodium atrovirens, C. desiccatum var.leptophylloides, andC. leptophyllum, producing the same array of flavonoids and with similar yet distinctive somatic chromosomes, are closely related, andC. desiccatum var.leptophylloides is considered to be a distinct species,C. pratericola. Chenopodium desiccatum var.desiccatum and C.hians show reduced flavonoid complements and have similar karyotypes. Chromosomal and chemical evidence suggests that plants heretofore included inC. incognitum represent two biological entities, one similar toC. atrovirens and another identical toC. hians. Morphological and chemical data indicate thatC. subglabrum, C. cycloides, and C.pallescens are closely related. The flavonoid chemistry and morphology ofC. albescens suggest a close relationship toC. atrovirens andC. pratericola.