Giant fiber interconnections were examined in successful grafts between two posterior portions of earthworms (Eisenia foetida). Electrophysiological and histological results indicated that cell-specific interanimal connections were formed between the medial giant fibers (MGF) in these preparations. Similar interconnections were formed between the paired lateral giant fibers (LGF), except that these involved the apposed left and right LGFs. Establishment of reliable and biodirectional conduction of LGF and MGF spikes across the graft occurred 2-4 days and 7-14 days after grafting, respectively. Subsequently, conduction across the graft remained effective for at least 6 weeks. The results are consistent with a hypothesis that grafted giant fibers are joined by a direct electrical connection, involving either fusion or an electrical synapse.