We compared two floras compiled in Ames, Iowa: (1) an "historic" flora based on two published floras (1871, 1890) and on 1450 herbarium voucher specimens of plants collected in Ames between 1859 and 1899, and (2) a "current" flora compiled by us during recent fieldwork (1990-2000). Our goals were to determine 1) long-term changes in composition (i.e., the proportion of native species) over time, 2) long-term changes in the abundance of individual plant species over time, and 3) the extent of gains and losses of native and non-native plant species. We found that the proportion of native species declined over time from 83.5% to 71.2%. Native taxa had a greater tendency to decrease in abundance and a lesser tendency to increase in abundance than did non-native taxa (p :S 0.001). Furthermore, historically uncommon plant taxa (regardless of origin) were more prone to extirpation from the flora than were more abundant taxa (p :S 0.001). Of the 277 plant species that likely entered the Ames flora after 1899, 160 of them (57.8%) are nonnative including eleven aggressive invasive species: garlic mustard (A/liaria petio!ata (Bieb.) Cav. & Grande), Japanese barberry (Berberis thunbergii DC.), crown vetch (Coroni!!a varia L.), leafy spurge (Euphorbia esu!a 1.), Amur honeysuckle (Lonicera maackii (Rupr.) Herder), purple loosestrife (Lythrum sa!icaria 1.), Osage orange (Madura pomifera (Raf. ex. Sarg.) Schneider), white mulberry (Moras alba 1.), European buckthorn (Rhamnus cathartica 1.), multiflora rose (Rosa mu!tiflora Thunb. ex Murray) and Siberian elm (U!mus pumi!a 1.). We argue that more floristic inventory work is needed to facilitate continued analysis of human impact on the Iowa flora.