ABSTRACT Thlaspi alliaceum L. (Brassicaceae) is a weedy, annual, European species of ruderal habitats known from 14 states in the eastern USA. It first was collected in North America in 1947 in Pennsylvania, although most references cite a 1956 collection from North Carolina. The species now has been vouchered in 23 Pennsylvania counties. Populations frequently consist of thousands of individuals. Despite its frequency, Thlaspi alliaceum is underreported in published works on the Pennsylvania flora.