Dead leaves falling into streams are an important food source for many invertebrates. They are generally made more palatable and more nutritious if they are first colonized by aquatic hyphomycetes and other micro‐organisms. At least two mechanisms appear to be responsible for this conditioning effect: microbial production (addition of easily digested microbial compounds to the nutritionally poor leaf substrate), and microbial catalysis (conversion of indigestible leaf substances into digestible subunits by microbial enzymes). Different invertebrate species vary in their ability to take advantage of microbial conditioning. This appears to be influenced by their mobility, the range of their food resources and their ability to overcome defense mechanisms of leaf‐colonizing microorgani