Gap junctions have been described ultrastructurally between neurons and epitheliomuscular cells and between neurons and their processes in the hypostome, peduncle and basal disc of Hydra. All gap junctions examined in Hydra exhibit two apposed plasma membranes having a 2-4 nm gap continuous with the extracellular space. The gap junctions are variable in length from 0.1-1.6 micrometers and appear linear or V-shaped in section. Neuronal gap junctions in Hydra occur infrequently as compared to chemical synapses. Electron microscopy of serial sections has demonstrated the presence of adjacent electrical and chemical synapses (neuromuscular junctions) formed by the same neuron. In addition, multiple gap junctions were present between two neurons. This is the first ultrastructural demonstration of electrical synapses in the nervous system of Hydra. Such synapses occur in neurons previously characterized as sensory-motor-interneurons on the basis of their chemical synapses; these neurons appear to represent a type of stem cell characterized by having both electrical and chemical synapses.