North American prairie remnants and restorations are normally managed with dormant-season prescribed fires. Growing-season fire is of interest because it suppresses dominant late-flowering grasses and forbs, thereby making available light and other resources used by subdominant grasses and forbs that comprise most prairie diversity. Here we report a twofold increase in mean frequency and richness of subdominant species after late-summer fire. Stimulation of subdominants was indiscriminate; richness of prairie and volunteer species increased in species that flowered in early, mid-, or late season. Early spring fire, the management tool used on this site until this experiment, had no effect on subdominant richness or frequency. Neither burn treatment affected reproductive tillering of the tallgrasses Sorghastrum nutans or Panicum virgatum. Flowering of Andropogon gerardii increased 4-fold after early-spring fires and 11-fold after late-summer fires. These preliminary results suggest that frequency and species richness of subdominants can be improved by late growing-season fire without compromising vigor of warm-season tallgrasses.