Longitudinal body wall muscles in the third instar larva of the fruitfly, Drosophila melanogaster, were systematically examined for electrical and dye coupling. These muscle cells were found to be electrically coupled but rarely dye-coupled across the segmental boundary. The inter-segmental coupling coefficients between muscle #6s and muscle #7s across the segmental boundary were 0.33 +/- 0.09 (mean +/- S.D., n = 12) and 0.43 +/- 0.09 (n = 5), respectively, which are much larger than values previously reported in Drosophila but similar to those reported in the blowfly and hawkmoth. By contrast, the intra-segmental coupling coefficient between muscles #6 and #7 was smaller, 0.16 +/- 0.08 (n = 28). Other muscle cells which had apparent physical contacts with these longitudinal muscles were examined but were not electrically coupled to them. Nerve-evoked as well as miniature excitatory junctional potentials were found also electrotonically spread across the segmental boundary. The inter-segmental coupling between muscle #6s was not blocked by the gap junction inhibitors halothane or 1-octanol. Functional significance of this electrical coupling is apparently in coordination of larval body movements.