ABSTRACTA c‐type, carbon monoxide‐binding cytochrome was isolated and studied from an endophyticNostocsp. Enzymatic transfer of electrons from NADPH to this cytochrome was demonstrated in cell‐free preparations under dark, anaerobic conditions, probably mediated by NADPH‐cytochrome reductase or ferredoxin‐NADP reductase present in the extracts. This cytochrome, with an α‐peak at 548 nm, had physicochemical properties similar to the low potential c‐type cytochrome, cytochrome c549, previously described by other workers fromAnacystis nidulans.The participation of such c‐cytochromes in dark electron transport, possibly respiratory, of bluegreen algae is st