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Linguatula serrata is a cosmopolitan parasite whose intermediate hosts are cattle, goats, sheep, and other ruminants. The adult form is found in the nasal airways, frontal sinuses, and tympanic cavity of canines and felines, and it produces hemorrhages and breathing difficulties. To elucidate if L. serrata produces enzymes that are capable of degrading tissues from the intermediate host, proteolytic activities in larval products were studied. Using the zymography technique, one major protease was detected in parasite in vitro-released products with an approximate molecular weight of 75 kDa. This enzyme was inhibited with phenylmethylsulfonyl fluoride, suggesting that it is a serine protease, which was also shown to degrade type I collagen. The serine protease exhibited maximal activity at alkaline pH and temperatures varying from 37 to 45 degrees C. To gather evidence about the physiological roles of the enzyme, further biochemical and functional studies are suggested.