A floating sampler was used to collect periphyton in the Ohio River at Cincinnati. The rate of colonization of glass slides by diatoms was determined, and the periphyton and plankton diatom communities were compared. The density of live diatom cells on the slides reached 15,000 per sq mm in approximately 30 days. The periphyton diatoms were dominated by species of Nitzschia and Navicula, whereas the plankton diatoms consisted largely of species of Melosira and Cyclotella. Dry weights of scrapings from slides exposed 14 days ranged from 149.5 mg per slide (32.5 sq cm) in July 1965, to 2.7 mg per slide in December 1965. Ash-free weights averaged 16.2% of the dry weight. (Modified author abstract)