We examined the effect of fire on vegetation composition with the use of an experiment with four different fire frequencies (annual, 2-yr interval, 4-yr interval, and an intended control with no burning) over a 27-yr period in an infertile, old-field grassland at the Cedar Creek Ecosystem Science Reserve, located in Minnesota, United States. We measured the plant species' aboveground biomass in permanent plots in 1983, 1987, 1991, 2000, and 2010. None of these fire frequencies, even after 27 yr, had a large impact on the vegetation composition. The plant functional groups' responses to fire frequency were consistent with their respective dominant species. The most abundant C₄ grass, little bluestem (Schizachyrium scoparium Michx.), did not change in biomass with fire frequency over time. The biomass of the introduced, invasive grass, Kentucky bluegrass (Poa pratensis L.) decreased significantly with increasing fire frequency, but this decrease did not result in change to the rate and trajectory of vegetation change. Bush clover (Lespedeza capitata Michx.), the only legume in the community, strongly increased with more frequent fire, but only after 20 yr. Species richness and litter mass decreased significantly with increasing fire frequency. These small fire-induced vegetation changes contrast with large fire-induced vegetation changes in fertile grasslands. Management strategies using fire in infertile grasslands can lower Poa abundance; however, increased fire also decreased overall plant diversity.