We recorded 502 introduced plant species during an inventory of all 2 334 private properties in Puerto Ayora, the largest town on Santa Cruz Island and the Galapagos Islands (population > 10,000) which accounts for about half of the Galapagos population. A third of these species were new records for the Galapagos Islands, 73% were found only in cultivation, 18% were naturalized and not cultivated (in Puerto Ayora), and 9% were found in cultivation and naturalized (escaped). Seventeen species previously known to occur only in cultivation in Galapagos are now considered escaped. A number of species known to be naturalized in the humid highlands of Santa Cruz do not appear to be able to reproduce in Puerto Ayora's drier climate. To determine which introduced species should be a priority for control or eradication, the potential invasiveness of each species was assessed based on their distribution and behaviour in Galapagos and elsewhere. At least 13 species were considered potentially serious invaders that could be feasible to eradicate.