Nucleotides and Ca 2+ binding to a-actin prepared from ordinary skeletal muscle of carp Cyprinus carpio was studied. When bound Ca 2+ was removed with ethylenediaminetetraacetic acid, carp a-actin denatured more rapidly than chicken a-actin. Kinetic studies of the denaturation process showed that in the absence of divalent cations, the binding constants of ATP to carp and chicken actin were 5.0 ¥ 10 4 /M and 1.2 ¥ 10 5 /M, respectively. Competitive binding of Ca 2+ between actin and 8-amino-2-[(2-amino-5-methylphenoxy)methyl]-6-methoxyquinoline-N,N,N′,N′-tetraacetic acid (Quin 2) showed that affinity of Ca 2+ for carp actin was also lower than that for chicken actin by a factor of 1.6. These results indicated that carp actin could relatively easily denature due to the low affinities of these ligands. Enthalpy changes upon ATP binding to carp and chicken actin were -65 kJ/mol and -110 kJ/mol, respectively. Thermodynamic analyses of our results revealed that the entropy change associated with ATP binding to carp actin was significantly smaller than that to chicken actin, suggesting that structural stabilization upon ATP binding was less effective in carp actin.