A fundamental study of the application of marine microalga to the aqueous-phase separation of toxic heavy metals was carried out. The biosorption characteristics of cadmium and lead ions were determined with marine microalga, Heterosigma akashiwo (Hada) Hada, which was inactivated by steam sterilization. A simple metal-binding model was used for the description of metal-binding data. The results showed that the biosorption of bivalent metal ions to H. akashiwo was due to monodentate binding to two different types of acidic sites: carboxylic- and phosphatic-type sites.