Nearly a century of fire suppression has changed fundamental aspects of the structure and functioning of many fire-adapted forest ecosystems. Many studies of these forests focus on the restoration of overstory structure and forest-floor fuels through reintroduction of fire or by mechanical means, without considering other important ecosystem attributes. In this study, I use 20 years of data from permanent plots in mixed-conifer forests of Sequoia and Kings Canyon National Parks, California, to explore changes in understory plant diversity and structure following reintroduction and repeated use of prescribed fire. Data on overstory structure, fuel loading, fire severity and heterogeneity, and the richness and abundance of plant growth forms were collected on 51 permanent plots for as many as 20 yr after treatment. Nonmetric multidimensional scaling showed distinct compositional changes over time in first- and second-entry burns despite considerable within-treatment variation. Ten years after fire, burned plots supported more than twice as many species as controls, with first-entry plots showing a nearly threefold increase in richness by year 20. Nonnative species occurred in only three plots at low abundance. Shrubs showed greater increases in richness in burned than in unburned plots, with burned plots supporting four to five times as many species as controls 5-10 yr after burning. Total understory cover increased significantly in first-entry plots, but exceeded that of the controls only after 20 yr; cover was dominated by perennial forbs and shrubs, but was highly variable among plots. In contrast, in second-entry burns, increases in plant cover were not significant (although post-treatment sampling extended to only 10 yr). Higher severity fires, regardless of treatment, led to greater numbers of species and to greater plant cover. Species richness was negatively correlated with burn heterogeneity in first-entry burns, but positively correlated with heterogeneity in second-entry burns. Long-term observations, which are rare in similar natural ecosystems, suggest that reintroduction of fire can gradually enhance the diversity and abundance of understory species. Moreover, repeated burning, which may be necessary to achieve structural and fuel-reduction objectives, does not appear to have detrimental effects on diversity and may enhance the distribution and abundance of species that have been affected by nearly a century of fire exclusion. More than two decades of plant community data from Sequoia and Kings Canyon National Parks suggest that multiple resource and ecological objectives can be met through the reintroduction of fire. However, viewing fire as critical to ecosystem restoration also requires that fire is maintained as a frequent and spatially dynamic process on the landscape.