SUMMARY1 Biofilms, the accumulation of micro‐organisms, exoenzymes, and detritus particles on submerged surfaces, may change the quality of leaves and wood as sources of food and/or habitat for invertebrates. We examined the relationship between macroinvertebrate assemblages and biofilm development on leaves and wood in a boreal river.2 Arrays of white birch ice‐cream sticks and sugar maple leaves were placed at fast‐ and slow‐current sites. Samples were collected periodically, and assayed for microbial biomass (ATP, ergosterol, chlorophylla) and macroinvertebrate colonizers.3 Microbial biomass and macroinvertebrate density were consistently greater on wood than leaves. Taxon richness was similar for all substratum/current combinations, but taxon density (number of taxa m−2) was greater on wood. Macroinvertebrate density and taxon richness correlated with all microbial indicators when data from both substrata were pooled.4 Leaves did not support as great a density of invertebrates as wood perhaps because of their faster breakdown rate and truncated biofilm development. Greater stability and the potential for surface complexity may make wood a site of higher macroinvertebrate dive