Decomposing barley straw (Hordeum vulgare) and oak leaves (Quercus robur) have previously been shown to inhibit the growth of a limited number of algae and cyanobacteria. Bioassays were conducted on a range of algae and cyanobacteria to evaluate their relative sensitivities to litter-derived inhibitor(s). A range of sensitivities were found, including some species that were stimulated by litter-derived inhibitor(s). Susceptibility to decomposing plant litter did not appear to be related to general taxonomic or structural features since susceptibility differed widely, even amongst members of the same genus. A microcystin-producing strain of Microcystis aeruginosa was very susceptible to decomposing barley straw. No specific effect on cell structure or morphology could be attributed solely to the litter-derived inhibitor(s). Evidence suggested that cell division, rather than cell expansion, was slowed or inhibited. Bioassays using Euglena gracilis showed the inhibitory compounds were not derived from the phototransformation of litter decomposition products and were not acting primarily by inhibiting photosynthesis. Barley straw inhibited the growth of filamentous algae in a drainage channel and, subsequently, the channel was recolonized by macrophytes. A one-year treatment with barley straw inhibited algal growth, but this reduction in growth was not maintained when the straw was removed. After three-years treatment with barley straw, macrophytes retained dominance after the removal of the straw. No inhibition of algal growth was observed in a different straw-treated drainage channel. The implications for water management are discussed.