Polymerus americanus (Reuter) is a mirine plant bug that has been seldom collected since its original description from Texas in 1876. Additional localities in Texas are recorded, and collection data are given for three previous records from Mexico (Coahuila, Hidalgo, and Nuevo León). Records from the eastern United States, and most western records other than West Texas, are considered probable misidentifications of other mainly dark species of the genus; the record from New Mexico requires verification. Nymphs and adults were observed in ornamental plantings of autumn sage (Salvia greggii Gray; Lamiaceae) at seven locales in Trans-Pecos Texas (Brewster, Jeff Davis, Pecos, and Presidio counties). Second through fifth instars and adults of a first generation were present in mid-April; late instars of another generation were found in late September and mid-October. The discovery of autumn sage as a host of P. americanus was facilitated by the plant's extensive use as an ornamental in the arid Southwest. The previous lack of bionomic information on the mirid is attributed mainly to the difficulty of finding native populations of autumn sage, a plant that grows in remote canyons and on mountain slopes in limited areas of West Texas and northern Mexico. Botanical information on autumn sage, including the history of its propagation, is provided.