Abstract The role of Romanomermis culicivorax in the degradation of hemolymph proteins in infected fourth-instar Culex pipiens was evaluated. Electrophoresis of proteins from hemolymph of C. pipiens, infected and control, revealed no new proteins in the hemolymph of infected mosquito larvae. Protein patterns from homogenates of R. culicivorax were compared with those from infected hemolymph and no proteins of parasite origin could be detected in the host hemolymph. Rabbit antibodies to R. culicivorax did not react with hemolymph of infected C. pipiens, a further indication that parasite proteins were absent from the host hemolymph. Proteases were located on polyacrylamide gels by their ability to digest gelatin. Appreciable proteolytic activity was found in the hemolymph of control mosquitoes, whereas activity in the hemolymph of infected mosquitoes and homogenates of R. culicivorax was nearly undetectable. Quantitative estimates of protease activity were made with two substrates, dimethylcasein and hemoglobin. Protease activity in hemolymph from C. pipiens larvae, control and infected, were not significantly different. However, the activity estimated for homogenates of R. culicivorax was 100 times less than that for hemolymph of C. pipiens larvae.