The cell structure of Cyanophora biloba, sp. nov. and C. paradoxa Korschikoff was examined using scanning electron microscopy, freeze-fracture, and sectioned material. The new species description was based on differences in cell micromorphology, in the cell covering, and in cyanelle structure and number. Cyanophora biloba is characterized by having flat, bilobed cells, with each lobe containing a cyanelle. Each cyanelle invariably has a central constriction that probably represents an arrested cytokinesis. A longitudinal groove or furrow is present between the lobes. The flagella arc inserted ventrally and subapically, and the area near the flagellar insertio is flanked by two short folds. Both flagella bear fine fibrillar hairs. The nucleus is horseshoe-shaped, conforming to the cell shape, and is located in the posterior of the cell. The cell covering in C. biloba consists of overlapping vesicles situated underneath the plasma membrane, and each vesicle contains a thin plate. Freeze fracture revealed that the plates appear to have a crystalline arrangement of subunits. For comparative purposes, the surface architecture and cell covering of C. paradoxa were re-examined and some features were found to be new or slightly different than previously described. The cell covering of C. paradoxa also consisted of overlapping vesicles with plates, but the vesicles were smaller than those in C. biloba. In addition, the cell had an anterior furrow that can be partially covered by two long folds near the site of flagellar insertion.