Hill prairies are grasslands on pronounced slopes. Of the many hill prairies in Illinois, 61, with a combined area of more than 200 acres, were visited by the writer inthe course of this study. In Illinois, hill prairies occur on the exposed upper or brow slopes of the generally southwest- and west-facing bluffs east of the Mississippi River for most of the length of the state and on similar slopes along the Illinois River from Putnam County southward into Jersey County, where the valley of the Illinois enters the Mississippi valley. Hill prairies are present also along the Sangamon and Rock rivers. The observed flora of Illinois hill prairie was 394 species and varieties. Of these, 390 were vascular plants distributed in 209 genera and 70 families. The largest family was Compositae, with 26 genera and 71 species and varieties. Aster was the largest genus, with 12 species. Thirty species were of foreign origin.