The influence of the net-spinning caddisfly larva Stenopsyche marmorata on streambed substratum stability was examined in 14 riffles of 5 rivers in northern Hokkaido, Japan. In each riffle, 6 representative stones were measured and 7 variables were quantified for each stone: 1) ambient flow velocity, 2) ambient depth, 3) extent of embeddedness, 4) force required to initiate shifting of the stone parallel to the flow direction, 5) biomass of S. marmorata living on and under the stone, 6) wet mass of the stone, and 7) size of the stone. Regression analyses were used to examine which of the variables best predicted the coherent strength of a particle with the stream bed (F_C). Riffle-scale patterns derived from the means of 6 stones showed significant influences of biomass of S. marmorata/stone and embeddedness on F_C; however, the collinearity of these 2 variables hindered evaluation of their relative importance. At the stone scale, biomass of S. marmorata/stone was positively related to F_C when standardized for the degree of stone embeddedness. F_C of stones inhabited by S. marmorata at the maximum range of their biomass is predicted to be 50 to 260% greater than F_C of stones without S. marmorata. The effects of biological consolidation of substratum particles by S. marmorata probably are substantial, even in inherently heterogeneous natural environments.