This paper describes four decades of in situ vegetation change in a post-agricultural site in southern New England where the role of the introduced vine Celastrus orbiculatus has drastically modified the course of vegetation development. An initial perennial forb-grassland community was replaced by two strikingly different physiognomic vegetation types-an early hardwood forest and a vine community occupying the same site 40 yr later. This ecological phenomenon was unpredictable since Celastrus was not recorded at the site until the 1970s. Within the young hardwood forest dominated by Prunus serotina and Acer rubrum a few old field species (five) persist whereas 17 forest herbs have become established. Typical forest shrubs are rare or absent. The Celastrus-dominated vine community has suppressed or eliminated pre-existing growth to form a relatively stable community where a forest vegetation is the regional vegetation type. Celastrus also appears to be facilitating a native vine (Vitis labrusca) by forming a ‘ladder’ for its advance. Although A. rubrum and Quercus spp. could ultimately dominate the forest community the contiguous and aggressive nature of Celastrus makes predictions questionable. This long-term study highlights the unpredictable nature of old field succession and the role of an exotic species in threatening the natural vegetation development.