Collections of vascular plants from nine study stations along the middle Mississippi River sand and mud flats in southern Illinois and southeastern Missouri were made over three growing seasons. Of 187 taxa recorded, the Gramineae, Euphorbiaceae, Compositae and Cyperaceae were, respectively, the most widespread and abundant groups. Floristic similarity coefficients of nine sites along 30 miles of shoreline ranged from .189 to .803. Line transects through representative communities indicated that six to seven species comprised 83 to 87 percent of the total relative frequencies of species encountered. Composition and distribution of river flat flora are greatly influenced by proximity of seed source, site habitat diversity, frequency and season of flooding, and chance dispersal of disseminules by water.