The morphology of the pectoralis major muscle and its antagonist, the supracoracoideus muscle, of the Chinese quail was studied in comparison with the Japanese quail, in relation to behaviourial characteristics. The actomyosin ATPase reaction after alkaline and acid preincubation reveals two fibre types. The indirect immunofluorescence, using specific antibodies against ‘slow’ (twitch) myosin from the human vastus lateralis muscle, provokes a weak reaction. All fibres observed show focal, monoaxonal ‘en plaques’ endings, typical of the phasic motor system. For the two quail species, the muscle structure seems adequate in relation to the movement characteristics, meaning muscle activity of migratorys, flying long di