Fire effects on structure, composition, and species diversity in canopy, shrub/sapling, and ground-cover strata were examined following three burns over an eight-year period in a small (6 ha), isolated flatwoods remnant in south-central Illinois. Prior to fire treatments, size-class distribution patterns for trees indicated two species groups, one comprised of oaks dominating the larger size classes, particularly Quercus stellata, and a second group of non-oak species mostly from the small-to-medium size classes. Ground cover was sparse, comprised mostly of tree seedlings and woody vines, and shade-intolerant herbaceous species were absent. Following three burns, total tree density (stems ≥ 6 cm) declined 26% from 465/ha; however, basal area increased from 24.7 m²/ha to 25.6 m²/ha as tree mortality mostly was confined to small-diameter stems. Acer saccharum was the only tree species not to decline in density in the fire-treatment area. While 15 of 20 species in the shrub/sapling stratum declined in density in the fire-treatment area, total stem density increased 170% from 7,133 to 19,227 stems/ha due to increases of Rubus allegheniensis and Sassafras albidum. Overall ground-cover species richness increased 81% following the three burns from 42 to 76 species while species density nearly tripled from 3.2 to 9.4 species/0.5-m². This increase included many characteristic flatwoods species; however, there also was an increase in ruderal species. Perennial forbs comprised the physiognomic class with greatest frequency following fire treatment and the most species-rich group; other groups that increased greatly in frequency and species richness were biennial and annual forbs and perennial sedges.