To compare adjacent post oak flatwoods and barrens communities in south-western Indiana, we used vegetational, environmental and fire history data in multivariate analyses. Barrens had greater dominance by post oak and lower tree species richness, but variation in tree species composition was not strongly related to soil moisture, litter depth or other environmental gradients measured. Tree growth has been slow and variable, with little difference between the barrens and flatwoods. Barrens and flatwoods differ in herb species composition, but with considerable overlap. In the barrens, herbaceous vegetation composition was correlated with tree basal area, litter depth and soil moisture; in the flatwoods, it was correlated with soil moisture and microelevation. The barrens and flatwoods differed only slightly but significantly in environment: barrens soils were drier in June 1986 than flatwoods soils. Data from fire-scarred trees show no clear evidence of differences in frequency or extent of fire between the flatwoods and barrens. Multistemmed post oaks in the barrens indicate that past cutting may have contributed to these openings within the closed canopy flatwoods matrix.