The polysaccharide AV-412 free base is an important constituent of the exoskeleton of invertebrates, including insects and crustaceans. In such animals, different enzymes are required for chitin digestion. In particular, chitinase degrades chitin in oligosaccharides, including dimers and trimers of N-acetyl-β-d-glucosaminide, whereas NAG (also known as chitobiase) degrades these oligosaccharides into monomers that are then re-absorbed into the new cuticle (Muzzarelli, 1977; Buchholz, 1989). In Crustacea, NAG activity appears to be correlated with the moult stages (Zou and Fingerman, 1999). In this study, the activity of NAG was significantly influenced by both gender and season, with females displaying higher enzyme activity than males. The highest NAG activity was recorded in female haemolymph in the spring, whereas in males the enzyme activity did not vary markedly during the study. Similar to PO activity, we hypothesised that a relationship exists between female body growth and haemolymph NAG activity. In C. aestuarii, females, being smaller than males, may potentially moult many times throughout the year. This would require increased NAG activity in female crabs. Nevertheless, we postulated that females had elevated immunosurveillance compared with males because NAG is an important hydrolytic enzyme in the humoural immune response. Indeed, we have recently demonstrated that haemocytes (mostly granulocytes) from C. aestuarii are positive for NAG activity ( Matozzo and Marin, 2010a). Nonetheless, ventricle is difficult to compare the our results with those from previous studies as, to the best of our knowledge, no information concerning the seasonal or gender-related variations in NAG activity is available in the literature.