There is growing evidence for impacts of small stream organisms on the transport of fine sediments. However, such impacts on the transport of coarser sediments remained unconsidered. Therefore we studied whether a silk-producing stream insect (Hydropsyche siltalai) consolidated gravel through silk fixed among gravel pieces. We exposed gravel for 2 months in a stream along a gradient of Hydropsyche abundance and measured the critical shear stress for this gravel in a laboratory flume. The critical gravel shear stress was very significantly (p=7.7¿10-11) related to a variable (overall biomass) describing gravel consolidation by Hydropsyche silk. We showed that Hydropsyche in a range of abundance typical of many streams increases critical shear stress for gravel by as much as a factor of 2. Surface gravel arrangement (i.e., the spatial distribution and orientation of different gravel sizes and forms) had a less significant impact on critical shear stress, while other sediment variables so far considered in erosion had no significant impact. Future studies on the transport of solids and on habitat conditions should consider the high potential of stream invertebrates to consolidate gravel. ¿ 1999 American Geophysical Union