To validate the utilization of organic matter, mainly composed of cellulose derived from mangrove trees by bivalves, we analyzed the cellulase activity in blood cockles (Anadara granosa) in the Mantang Mangrove Forest Reserve, Malaysia. In an agar plate assay, unstained circles due to cellulase activity were clearly detected in the extract of blood cockle digestive organs. Additionally, SDS-PAGE zymographic analysis revealed intensive active bands at 9.5 kDa and weak smear bands above 24.5 kDa. These results clearly showed that the blood cockle, the dominant mangrove bivalve species, possesses cellulase activity used to break down cellulose derived from mangrove trees. The present paper first reports the occurrence of the cellulose breakdown function in a mangrove bivalve, blood cockle, via biochemical analyses and suggests the possible ecological function of this species in the mangrove food chain.