The introduced North American signal crayfish Pacifastacus leniusculus (Dana) has replaced the native noble crayfish (Astacus astacus) in many European freshwaters and can be considered a new component of these ecosystems. The 2 species are apparently similar in many respects, but their ecological equivalence is uncertain and has been little investigated, especially at the whole-lake scale. We compared the effects of the 2 species on the abundance, species richness, and composition of littoral macroinvertebrate assemblages in a set of small- and mediumsized boreal lakes, which included 8 lakes with noble crayfish, 8 lakes with signal crayfish, and 8 lakes without crayfish. We collected semiquantitative littoral macroinvertebrate samples with a kick net from 3 replicate sites in each lake. The abundance of invertebrates did not differ significantly among the 3 lake categories, but lakes with crayfish had lower species richness than lakes without crayfish. Mollusk taxa, in particular, were fewer in lakes with crayfish. Assemblage composition also differed between lakes with and without crayfish. However, macroinvertebrate species richness and composition did not differ between lakes with signal or noble crayfish, indicating that the 2 crayfish species are ecologically equivalent with respect to their effects on shallow, littoral invertebrate assemblages of boreal lakes.