Isometric twitch of lateral musculasture by electrical stimulation was examined using rainbow trout Salmo gairdneri 105-365g in weight. Fish were restrained on an experimental apparatus, and perfused through the mouth with oxygenated fresh water. The suitable electrode positions at which electrical stimulation evoked a maximal twitch tension and a normal tail beat were selected: an anode and a cathode were inserted directly into the muscle or placed on the body surface, at the lateral line between the posterior limit of the operculum and the anterior limit of the base of the dorsal fin, respectively. The twitch tension increased with the elevation in stimulating voltage to a maximum at around 10-30 V by 2 ms square pulse using surface electrodes, and at around 4-20 V by 3 ms square pulse and at around 5-35 V by 0.005 ms square pulse using needle electrodes, and maintained approximately maximum at a stimulating voltage of up to at least 50 V. Therefore, the supramaximal stimulation has been shown to be applicable to development of a maximal twitch tension.