A study has been made O'f the enzymic degradation of chitin and the sodium salt of chitin sulphuric acid by a chitinase prepared from the intestinal tract of the snail, Helix aspersa. It is shown that, in a citrate-phosphate buffer, the pH for optimum activity is 4.8 for both substrates. Chitin, prepared from both lobster cuticle and fly puparia, is broken down by snail chitinase to N-acetylo- glucosamine, although there is alsO' a trace O'f o-glucosamine. The sO'dium salt O'f chitin sulphuric acid alsO' breaks dO'wn to N-acetyl-D-glucO'samine and o-glucosamine. In both cases N-acetyl-D-glucO'samine was isolated. The chitinase is without action O'n chitin nitrate. Acid hydrolysis O'f both chitin nitrate and the sO'dium salt O'f chitin sulphuric acid leads to' the fO'rmation O'f o-glucosamine, which has been isolated as 2-hydrO'xynaphthylidene glucO'samine. All attempts to' phO'sphorylate chitin were unsuccessful. The structure O'f chitin is discussed.