AbstractSensitivity of cultured chick myotubes to alkaline earth metal ions was investigated by recording contractile isometric tension through a semiconductor transducer. The myotubes were obtained by culturing myoblasts of chick embryo breast muscles, and skinned chemically before physiological experiments. Contractions developed in response to Ca2+in a bathing medium higher than 3 × 10−7M and reached maximum at 1 × 10−5M. Sr2+was less effective than Ca2+; the threshold concentration was 1 × 10−5M and the tension reached maximum at 1 × 10−3M. Ba2+was the least effective among the three alkaline earth metal ions; only one fifth of the Ca2+‐induced maximum tension was attained at 1 × 10−3M. The sensitivity was similar to that of the mature pectoral muscle fiber, a fast twitch muscle fiber, rather than that of the anterior latissimus dorsi, a slow tonic muscle fiber. The sensitivity was shown to be dependent on its troponin C by replacing it with troponin C from the mature pectoral or cardiac muscle. This indicates that TnC of a fast‐muscle type is expressed in the cultured chick myotube as in the mature pectoral muscle. The contractile apparatus was thus shown to be well developed in the cultured myotube with characteristics similar to the mature fast t