Competition can be an important structural factor within macroinvertebrates such as the two common freshwater herbivores, the snail Lymnaea peregra and the mayfly Cloeon dipterum. In our study, mortality increased for both species due to intraspecific competition, and furthermore, snail growth decreased with increasing snail densities. Interspecific competition did not affect mortality but instead resulted in a change in microhabitat. However, we also found a positive effect of mayfly density on snail growth, most likely due to indirect commensalism. We did not find any density-dependent effect of grazing on periphyton, probably due to interfering interactions between grazers. However, there was a significant difference in periphyton biomass due to grazer composition. Irrespective of grazer density, when the two species coexisted they decreased the periphyton biomass significantly compared to both single-species treatments. This we believe is the joint action of facilitation and interaction. Our results suggest that competition can be an important structuring factor in macroinvertebrate communities and that species composition can be significant for ecosystem processes.