Gap junctions between three different cell combinations in Limulus are studied with thin-section, lanthanum-tracer, and freeze-fracture techniques. Midgut epithelial (E) cells form E-E gap junctions. Reserve (R) cells adjacent to the outer surface of the midgut form R-R gap junctions. E-R junctions are formed when R-cell processes extend through a basement membrane and contact E-cells. All junctions possess parallel membranes with 30 Å gaps, polygonal substructure when treated with lanthanum and sectioned en face, and electron-dense 25 Å dots in subunit centers. In freeze-fracture replicas only E-E and R-R junctions can be identified. All junctions possess aggregates of 125 Å particles in each membrane. Particles are observed only on outer fracture faces and, like the complementary pits, are not highly patterned. In both particle centers and pits, 30 Å spots of platinum are seen. The substructure of gap junctions between cells of the same type appears indistinguishable from that between different cell types.