Effects of ambient levels of snail grazing on rates and direction of benthic algal succession were investigated in experimental chambers located near two lakes. Benthic algal environments were manipulated by introducing snails (15 individuals/m2) to flow-through experimental systems (for 26-28 d) where clay tiles were colonized with algae from littoral lake water from Douglas Lake, Michigan, and Kentucky Lake, Kentucky. We found that algal succession was not always evident in ungrazed conditions; but when successional changes were evident, many different growth forms were observed among early and late succession species. Early succession species were typically non-motile unicellular or colonial growth forms, whereas late succession species were motile unicellular or filamentous diatoms and cyanobacteria. Grazing by the snail Elimia livescens arrested succession of non-diatom algae by selectively removing the overstory of filamentous cyanobacteria. However, grazing by E. livescens enhanced succession among diatoms by selectively grazing the early succession diatoms and leaving small motile late succession species. Whether grazing enhanced or arrested succession, or changed the direction of succession, was dependent upon growth forms of early and late succession algal species, which varied among habitats and corresponding species assemblages.