Structural stability and electrochemistry of chlorella ferredoxin immobilized in a cast film of molecular bilayer membranes of a biological lipid, dimyristoylphosphatidylcholine (DMPC) on a quartz substrate or a pyrolytic graphite electrode were examined, A specular reflectance spectrum of the film showed that the [2Fe-2S] cluster in ferredoxin retains its original structure even in an air atmosphere. This indicates that the lipid bilayers provide suitable microenvironments for a stable immobilization of the protein. Cast films of ferredoxin+DMPC on a graphite electrode gave well-defined, nearly reversible redox waves with a formal potential of -0.59 V vs. Ag/AgCl, which is close to the reported value at promoter-modified electrodes, indicating that the electrochemical communication is from native ferredoxin.