The fine structure of the dorsal giant fibre septa of earthworm (Lumbricus terrestris and Eisenia foetida) was studied by thin sectioning. E-PTA, BIUL and ZIO impregnation were carried out to characterize the various membrane specializations found. A septum, which is formed by two axon membranes only ca. 7 nm apart, is a heterogeneous structure showing a number of different specializations (intermediate junctions, dense projection-like humps, septate regions, vesicles quite often associated with a widened intercellular gap and membrane thickening). Septal areas referred to as septal complexes are of particular interest. They comprise up to 20% of septal cross-sections and are characterized by membrane appositions (diameter ca. 25 nm) which bridge the septal gap, protrude 16-25 nm into the respective axoplasms, and form a more or less hexagonal array with a centre-to-centre spacing of ca. 33 nm (ca. 29 nm after E-EPTA treatment) in en face sections. We interpret the septal complexes as gap junctions, i.e. sites of electronic coupling, and emphasize their unusually large dimension. The vesicles found at the septum could not be stained by ZIO treatment.