Abstract The effects of trematode infection on the hemolymph proteins of mayfly naiads (Hexagenia recurvata) and alderfly larvae (Sialis sp.) were studied using polyacrylamide gel disc electrophoresis. One-to 14-day experimental infections with Allopodocotyle lepomis cercariae were associated with depletion of total hemolymph protein in both species. Protein was lost from all fractions of Hexagenia hemolymph except the fastest migrating one. Two fractions of Sialis hemolymph which showed significant reduction in protein contained tyrosinase activity which also decreased during infection. Tyrosinase was also located in one-fifth to one-half of the hemocytes of both insects. These results suggest that the tyrosinase involved in the melanization of trematode larvae within Hexagenia is confined to the hemocytes, whereas in Sialis the hemolymph tyrosinase may also play an important role.