The toxicity of tin on the excitation-contraction coupling of skeletal muscle fibers was studied in “nonpolluted” and in “polluted” crabs. In fibers obtained from “non-polluted” crabs living in artificial seawater (ASW): (i) the resting membrane potential of muscle fibers decreased irreversibly from the control value of −67.0 ± 3.1 to −59.8 ± 3.9 mV after 10 min of exposure to tin (10 −4 m); (ii) the action potential rapidly (5 min) and irreversibly changed from an all-or-none type to a graded response in the presence of tin; (iii) the twitch tension decreased in 4 min from a mean value of 0.92 ± 0.29 kg·cm −2 in ASW to a new value of 0.50 ± 0.32 kg·cm −2 in ASW + tin. The crabs which were housed in ASW + tin did not survive beyond 15 days. In the fibers from these “polluted” crabs: (i) the resting membrane potential fell to −59.6 ± 2.4 mV after 18 hr of exposure of the crabs to ASW + tin; the value after 24 hr was −51.5 ± 3.9 mV and the membrane potential was stable at that value until the ninth day of life in ASW + tin; (ii) the action potentials were all of the graded type after 18 hr in ASW + tin; (iii) the twitch tension was 0.61 ± 0.21 kg·cm −2 after 24 hr. It was still 0.55 ± 0.30 after 9 days. The potassium contracture decreased from the control value of 2.7 ± 0.1 to 0.97 ± 0.52 kg·cm −2 after 4 days of life in ASW + tin. At the concentration of 10 −4 m, tin is a toxic lethal substance which depresses electrical and mechanical activity of the crustacean skeletal muscle fiber.