During the course of screening diarrhetic toxin in bivalves, we observed that ether extract of toxic mussels caused the conversion of oxyhemoglobin to methemoglobin. The ether extract was fractionated by liquid-liquid partition method. Among the preparations, the acidic fraction had the action of converting oxyhemoglobin to methemoglobin. Okadaic acid, which is a diarrhetic toxin in bivalves, did not show such an effect. From these results, it is considered that the conversion of oxyhemoglobin to methemoglobin is caused by some substances in the acidic fraction of ether extract of toxic mussels other than okadaic acid.