Many freshwater streams are predominantly heterotrophic, the energy source for all trophic levels of the biota being derived from allochthonous plant litter that is degraded by saprotrophic micro-organisms. Leaves falling into streams have most of their soluble constituents leached out within twenty-four hours of submersion. Few of the resident terrestrial saprotrophs survive in the aquatic environment and the leaves are rapidly colonised by freshwater fungi, particularly aquatic hyphomycetes (Chamier & Dixon 1982a; Chamier et al. 1984). These fungi produce all the enzymes required to degrade the structural polysaccharides of leaf cell walls and those of resident terrestrial fungi. Some species can degrade lignified tissue (Chamier & Dixon 1982b; Chamier 1985). Little is known of the enzymic capabilities of freshwater bacteria.