TheDrosophila giant axon pathways ‘cervical connective — thoracic indirect flight muscles’ were studied by a combined electrophysiological and genetic analysis. A functional coupling of the left and right giant axon pathways was revealed by intracellular recordings of electrical responses of the thoracic indirect flight muscles, when evoked by electrical stimulation of cervical connective (Fig. 2). This functional coupling was demonstrated in wild-type flies and in flies of the single gene, temperature-sensitive paralytic mutation,para ts . The functional coupling was evident also in selected bilateral gynandromorph flies, mosaics for thepara ts mutation (Fig. 1), even at restricted elevated ambient temperature (Tables 1–3). Analysis of neurally evoked electrogenic muscle responses of wild-type flies, following injection of picrotoxin, verifies the notion that both the dorsoventral and the dorsolongitudinal flight muscles share a common activating pathway (Fig. 3). Picrotoxin application to gynandromorph flies demonstrated the existence of neuronal elements additional to the giant axon pathways, that evoke the indirect flight muscles in response to cervical stimulation (Figs. 4, 5). An unexpected finding was the poor correlation between the mosaic external phenotype of the gynandromorph flies ofpara ts mutation and the genotype of neural pathways activating their thoracic flight muscles, as evidenced by the intracellular recordings.