In the lumen of sarcoplasmic reticulum (SR), a greater part of the Ca²⁺ ions is bound to the calsequestrin 2 (CASQ2), a 45-55 kDa protein, which is anchored to ryanodine receptors (RyR) via triadin and junctin. In addition to its role in Ca²⁺ storage, CASQ2 also modulates the activity of RyRs, and is thus an essential part of the cardiac e-c coupling. Although the majority of Ca²⁺ needed to cause contraction in trout (Oncorhynchus mykiss) cardiac myocytes comes from extracellular space through sarcolemmal Ca channels, the SR Ca²⁺ release still plays a physiologically significant role in e-c coupling, especially in fish which have been acclimated to low temperature (4°C). Indeed, Ca²⁺ stores of the trout SR are more than double in comparison those of the mammalian cardiac SR. Therefore, we hypothesized that large Ca²⁺ stores and relatively weak Ca²⁺ -induced Ca²⁺ release of the trout heart are due to strong buffering of Ca²⁺ by CASQ2. To this end, the CASQ2 was cloned, sequenced, and the expression of CASQ2 transcripts were determined by quantitative PCR in atrium and ventricle of cold (4°C) and warm acclimated (18°C) trout. The expression of CASQ2 mRNA relative to mRNA of the DnaJ gene was about three times higher in cold-acclimated than warm-acclimated trout in both atrium (0.86 and 0.31, p = 0.002) and ventricle (0.73 and 0.20, p = 0.025). This suggests that temperature-dependent regulation of SR CASQ2 is involved in thermal acclimation of cardiac e-c coupling in the trout heart.