1. beta-N-acetylhexosaminidase isoenzymes from the gastropod, T. cornutus, were purified and their properties studied. 2. The two isoenzymes, designated A and B were separated by DEAE-Sephadex column chromatography and further purified by CM-cellulose, Concanavalin-A-Sepharose-4B and Sephadex G-200 column chromatography. 3. beta-N-Acetylhexosaminidase A and B were purified 416 and 208 fold, with yields of 10.6 and 5.1%, respectively. 4. The two isoenzymes appear homogeneous on polyacrylamide gel electrophoresis, with the A form migrating faster towards the anode than the B form. 5. The purified isoenzymes are virtually free of all other common glycosidase contaminations. 6. The apparent molecular weight of both beta-N-acetylhexosaminidase A and B is about 100,000 when estimated with gel filtration column chromatography and the pH optimum for both is 4.0. 7. Both beta-N-acetylhexosaminidase isoenzyme activities are stimulated by Cl-, Br-, F-, I- and NO3-, and inhibited by Hg+, Ag+, Fe3+, N-acetylglucosamine and N-acetylgalactosamine. 8. The Km values of beta-N-acetylhexosaminidase A and B for the substrate p-nitrophenyl-beta-2-acetamide-2-deoxy-D-glucopyranoside were 2.9 and 3.2 mM, respectively.