Two distinct endopeptidases were detected histochemically in the cercariae of Diplostomum pseudospathaceum: a serine proteinase occupying the ceca and a cysteine proteinase occupying the penetration glands. The former was relatively resistant to formaldehyde, required either Ca2+ or Mg2+ for its stability, and was sensitive to organic fluorophosphate inhibitors but insensitive to thiol-blocking agents. The latter enzyme required both reducing and divalent cation-complexing agents for its full activity and was sensitive to formaldehyde, thiol-blocking agents, and diazonium salts. Both enzymes hydrolyzed a number of N-blocked L-aminoacyl-, and N-blocked L-peptidyl-naphthylamides bearing L-arginine at the P1 subsite. The pH optima for hydrolysis of the substrates were 8.0 for the serine proteinase and 7.6 for the cysteine proteinase.