Despite their physiological significance in immune and growth-controlling processes in plants and animals, no chitinolytic enzyme has been identified yet at the molecular level in Lophotrochozoa, one of the major clades of bilaterian animals. Here, we report the cloning and the characterization of a singular chitinase homologue from the bivalve mollusc Crassostrea gigas (Cg-Chit). This protein displays a modular structure including a conserved catalytic domain attached to a peritrophin-A type chitin-binding domain and an unconventional C-terminal hydrophobic sequence acting as a potential membrane anchor domain. Gene expression profiles monitored by quantitative RT-PCR in different adult tissues and during development support for the first time the involvement of such a protein in early embryonic development. Furthermore, Cg-Chit encoding gene was transcriptionally stimulated in haemocytes in response to either bacterial or LPS challenge. This suggests that Cg-Chit plays an important role as an immunity effector in molluscs.