Parasites can alter a wide range of host behaviors resulting in changes in organismal interactions and ecosystem processes. One of the most important behaviors that controls food web dynamics is herbivore grazing because an alteration in grazing behavior leads to changes in trophic dynamics and ecosystem processes by changing the abundance and diversity of primary producers. To test whether parasite load can alter host grazing levels and choices, feeding trials were conducted using the keystone species, the rusty crayfish Faxonius rusticus (Girard, 1852), grazing on a selection of macrophyte species. The rusty crayfish is a keystone species because its grazing significantly alters the abundance of macrophytes in freshwater ecosystems. We used a total of 165 wild-caught, naturally-infected crayfish individuals with a wide range of parasite loads by species of the digenetic trematode Microphallus Ward, 1901. Crayfish were presented with 1 g each of the macrophytes Elodea canadensis (Michaux), Ceratophyllum demersum (L), Chara sp., and Potamogeton richardsonii (A. Benn.) in a 23-hr foraging assay. Subsequently, crayfish were dissected, and parasite loads were calculated. Mixed models were then utilized to determine how parasite load affected consumption. As infection of Microphallus increased in the crayfish hepatopancreas, consumption of all four macrophytes significantly decreased. Melanization of Microphallus spp. within the hepatopancreas, the immune response to. infection, did not significantly reduce crayfish macrophyte consumption. These results indicate that macrophyte consumption in the crayfish was affected by Microphallus. This impact on crayfish grazing could alter macrophyte abundances in aquatic ecosystems. Because of the many ecosystem functions macrophytes play, an alteration in their abundances could lead to community-level ramifications by impacting nutrient flow and organismal abundances in aquatic ecosystems.