Parasites can have density- and trait-mediated effects on their hosts, which can have indirect effects on ecosystem function and structure. These effects of parasites may alter the ecological impacts of invasive species, but few studies have evaluated their relative importance. We explored how a parasite alters the ecosystem impacts of the invasive rusty crayfish (Faxonius rusticus) via both changes in crayfish density and changes in crayfish traits. Ecosystem impacts of F. rusticus are well documented with dense populations causing greater impacts (density-mediated) and more active crayfish causing greater impacts (trait-mediated). In 2019, we discovered the outbreak of a microsporidian parasite in F. rusticus that was associated with a significant decline in the crayfish population in a Wisconsin lake. We also found that infected crayfish had lower activity levels than uninfected crayfish. Therefore, we hypothesized that microsporidian infection would reduce F. rusticus ecosystem impacts through a decrease in crayfish activity or feeding rate. We conducted a 3-week mesocosm experiment to test the trait-mediated indirect effects of the microsporidium on ecosystem function and structure (i.e., number of macrophytes, macrophyte biomass, leaf litter breakdown, benthic metabolism) by varying parasite presence. We also varied crayfish density so that we could compare the strength of trait-mediated effects to a reduction in crayfish density. In support of our hypothesis, we found that tanks with infected crayfish consumed less macrophyte shoots and had lower benthic metabolism. Our results also suggest that this parasite is likely to cause density-mediated effects because crayfish mortality rates were higher in tanks with infected crayfish. Behavioral and macroinvertebrate data are still being processed. Overall, we found invasive crayfish impacts can be reduced by the presence of a pathogen through both trait- and density-mediated indirect effects.