The frequency and specificity of associations of vorticellids with planktonic algae were examined and compared to associations with macrophytes and artificial substrates. Most associations were nonspecific. However, Vorticella monilata was found exclusively as an epibiont on Anabaena flos-aquae (Cyanophyta). Over two-thirds of the A. flos-aquae filament masses examined had attached vorticellids. Vorticellids were found in large numbers and were associated with increased motility of the filament masses observed. Additionally, Vorticella microstoma was found on nearly one-half of Microcyctis aeruginosa colonies collected. This association was not specific inasmuch as V. microstoma was common on several other substrates. Advantages of such associations of vorticellids and planktonic blue-green algae are discussed.