This investigation describes the ultrastructure of the atrioventricular node of Eptesicusfuscus. Two conducting cell types (nodal and transitional) are identified which differ in location, myofibrillar content, and types of intercellular junctions. Centrally located nodal cells display variable staining intensity and contain disorganized myofibrils which rarely form sarcomeres. Desmosomes and nexus-like junctions connect the nodal cells. Transitional cells, situated peripherally, exhibit distinct sarcomeres and are attached to the adjacent cells through desmosomes and underdeveloped intercalated discs. Longitudinal arrangement of the mitochondrial cristae is frequently seen in both cell types. In the connective tissue stroma, numerous capillaries (with micropinocytotic vesicles), axons, and possibly axonal terminals, some filled with vesicles, are observed. A large ganglionated nerve trunk is present on the nodal periphery. True nexuses and neuromuscular junctions are not observed. It is suggested that nodal cell types previously reported in different vertebrates under various names are merely simple variations of the two basic types of conducting cells – nodal and transitional. No interspecffic differences are observed between these cells of the big brown bat compared with those in other vertebrate