Mormyromast electroreceptor organs are the most numerous type of electroreceptor organs in mormyrid electric fish and provide the sensory information necessary for active electrolocation. Mormyromast organs and their primary afferent fibers have not been studied very extensively. Both morphological and physiological questions remain to be answered before the neural basis of active electrolocation in mormyrids can be understood. This paper examines four different aspects of the morphology of mormyromast organs and afferent fibers: 1) Mormyromast organs in the skin. The innervation patterns for the two types of separately innervated sensory cells in the mormyromast organ are described on the basis of silver-stained whole mounts of skin. The number of sensory cells per mormyromast organ increases linearly with fish growth for both types of sensory cells. 2) Relation between peripheral sensory cell innervated and central zone of termination for mormyromast afferent fibers. The afferent fibers arising from the two types of sensory cell in the mormyromast organ project to separate zones of the electrosensory lateral line lobe, as shown by using retrograde labeling with horseradish peroxidase. 3) Central trajectories and terminal arbors of mormyromast afferent fibers. These aspects of mormyromast fibers are described by using intracellular staining of individual fibers as well as whole nerve staining of an electrosensory nerve. 4) Fine structure of mormyromast afferent terminals in the electrosensory lateral line lobe. Afferent fibers make various synaptic contacts, including contacts of a mixed type, gap junction-chemical, onto a restricted class of granule cells. The fine structure is described based on electron microscopy of horseradish-peroxidase-labeled fibers. The results provide an anatomical base for current physiological studies on mormyromast afferent fibers.