Troponin C was isolated from the skeletal muscle of bullfrog (Rana catesbeiana), and its relative molecular mass was estimated to be 18,000 by SDS/polyacrylamide gel electrophoresis. In its amino acid composition, bullfrog troponin C was similar to that of the frog (Rana esculenta) but different from that of rabbit. Its ultraviolet spectrum was consistent with its amino acid composition. The ultraviolet difference spectrum of the Ca(2+)-loaded form vs. the metal-free form indicated that the single Tyr residue and some Phe residues in the bullfrog troponin C molecule were affected by the conformational change associated with Ca2+ binding. On electrophoresis in polyacrylamide gel in 14 mM Tris and 90 mM glycine, the metal-free and Mg(2+)-loaded forms migrated slower than the Ca(2+)-loaded form. The property is shared by rabbit troponin C but not parvalbumins or calmodulin. The ATPase activity of CDTA-treated myofibrils reconstituted with bullfrog troponin C showed the same Ca(2+)- and Sr(2+)-sensitivity as that of those reconstituted with rabbit troponin C. Bullfrog troponin C is, thus, physiologically the same as rabbit troponin C, in spite of several marked differences in their physicochemical properties.