Chitinase degrades chitin which comprises an important source of carbon and nitrogen in the marine environment. The aim of this study was to evaluate the population of chitinases in the marine sediments in southwest Japan. We collected marine sediments from eutrophic inner bay and offshore. Chitin-degrading bacteria were enriched from both sediments. Metagenomic DNA was isolated from the enriched chitin-degrading bacterial cell culture. At the same time, 25 chitin-degrading bacteria were isolated from the enriched culture. Partial fragments of chitinase genes were successfully amplified with degenerate primers designed for the glycoside hydrolase 18 family. We analyzed chitinase gene sequences of about 500 clones from metagenomic DNA prepared from chitin-degrading bacteria. Based on translated amino acid sequences, chitinases were grouped into five groups. Chitinases in groups II and III was most abundant and close to chitinase genes of several species of proteobacteria. On the other hand, chitinases in groups I, IV and V were unique and distinct from the known chitinases. These results indicate that the marine sediments used in this study contain diversity of chitinase genes.