AbstractThe sarcoplasmic reticulum (SR) was prepared from the ordinary white muscle of carpCyprinus carpioacclimated to either 10 or 30°C over 5 weeks, and its Ca2+‐ATPase and Ca2+uptake activities measured.The SR Ca2+‐ATPase activities of the 10 and 30°C‐acclimated carp were 0.2 and 0.1 μmol Pi/min.mg at a reaction temperature of 10°C, respectively, and 1.1 and 0.6 μmol Pi/min.mg at 30°C, respectively. The corresponding Ca2+uptake activities in the presence of sodium oxalate were 1.1 and 0.4 μmol Ca/min.mg at 10°C, and 2.1 and 1.5 μmol Ca/min·mg at 30°C, respectively. The break point in the Arrhenius plot of Ca2+‐ATPase activity was 9.2°C for the 10°C‐acclimated SR and 15.9°C for the 30°C‐acclimated SR. The activation energy of Ca2+‐ATPase for the 10°C‐acclimated SR between 10 and 40°C was 75 kJ/mol and similar to that for the 30°C‐acclimated SR between 16 and 40°C, 81 kJ/mol. The membrane fluidity of SR, which was determined with a fluorescence probe, Py(3)Py, was higher for the 10°C‐ than 30°C‐acclimated carp at any reaction temperature from 0 to 30°C. The peptide map in SDS‐gels of Ca2+‐ATPase with α‐chymotrypsin gave different patterns in the 40–70 kDa region between the two acclimated groups: the peptide of about 63k Da was specific to the 10°C‐acclimated SR, while the peptide of about 50 kDa to the 30°C‐acclimated SR. Tryptic peptide mapping also resulted in different fragmentation patterns in SDS‐gels for the two Ca2+‐ATPases. These results suggest that carp would modify the fluidity of SR membrane and the molecular structure of Ca2+‐ATPase to compens