A 56 kDa chitinase isozyme (PaChiB) was purified from the stomach of the silver croaker Pennahia argentatus. The optimum pH and pH stability of PaChiB were observed in an acidic pH range. When N-acetylchitooligosaccharides ((GlcNAc)n, n=2 -6) were used as substrates, PaChiB degraded (GlcNAc)4 -6 and produced (GlcNAc)2,3. It degraded (GlcNAc)5 to produce (GlcNAc)2 (23.2%) and (GlcNAc)3 (76.8%). The ability to degrade p-nitrophenyl N-acetylchitooligosaccharides (pNp-(GlcNAc)n, n=2 -4) fell in the following order: pNp-(GlcNAc)3≫ pNp-(GlcNAc)2 pNp-(GlcNAc)4. Based on these results, we concluded that PaChiB is an endo-type chitinolytic enzyme, and that it preferentially hydrolyzes the third glycosidic bond from the non-reducing end of (GlcNAc)n. Activity toward crystalline α- and β-chitin was activated at 124%-185% in the presence of 0.5 M NaCl. PaChiB exhibited markedly high substrate specificity toward crab-shell α-chitin.