Few data are available that compare long-term changes in burned and unburned prairie remnants in the eastern part of the tallgrass prairie region. To assess effects on species richness and composition, in 1996 we resampled three dry and four dry-mesic sand prairie remnants that were originally sampled in 1976. Three of these sites were managed with burning over the 20 y period and four were not, allowing a comparison of temporal change among burned and unburned sites. Mean richness of native species per plot increased only in burned sites and declined only in unburned sites. In contrast, richness of alien species increased only in unburned sites and declined only in burned sites. In unburned dry-mesic sand prairies, an increase in the alien grasses Bromus inermis and Poa pratensis was accompanied by a decline in the native grasses Schizachyrium scoparius and Stipa spartea, and the forbs Echinacea pallida, Helianthus rigidus and Coreopsis palmata. Railroad rights-of-way prairies bordering agricultural fields were most vulnerable to invasion of alien grasses. These findings underscore the need to use fire to prevent invasion of alien grasses and to maintain native species richness and composition of prairie.