Ontogenetic stage and density of consumers can affect the intensity of their roles within a community. These roles can be further complicated in a heterogeneous environment, where a consumer might have disparate effects in different microhabitat types. We conducted an enclosure experiment in a small temperate stream to test the differential ecological roles of adults and juveniles of the signal crayfish (Pacifastacus leniusculus) in 2 microhabitat types. In addition, we manipulated numbers of each stage to create a gradient of density (and biomass). Enclosures contained 1, 2, or 3 adults, or 4, 8, or 12 juveniles. Control enclosures lacked crayfish. We added leaf packs the 4 corners of the enclosures to provide a primary resource for crayfish and other invertebrates and to provide a detritus-based microhabitat. After 6 wk, we collected the leaf packs and a cobble-bottom (Surber) sample from the center of the enclosures. We used cobble-bottom samples to analyze the composition of the community in the cobble microhabitat. Leaf-pack breakdown rate was positively related to crayfish biomass for adults and juveniles. Leaf-pack invertebrate community composition and abundance were negatively affected by crayfish presence, but these results were independent of crayfish ontogenetic stage and total biomass. The composition of the cobble-bottom invertebrate community was not significantly affected by the presence of crayfish, regardless of density or ontogenetic stage. This result might have been the consequence of the cannibalistic tendencies of P. leniusculus or of possible indirect effects of crayfish (e.g., bioturbation or chemical cues). Our study is among the first to be done on the ecology of P. leniusculus in its native environment. The fact that our results do not agree with those found in other studies for this organism might reflect the fact that most of what we know about this organism comes from research done where it has been introduced.