Adaptation for life at different temperatures can cause changes in many aspects of an organism. One example is the expression of different protein isoforms in species adapted to different temperatures. The calcium regulatory protein cardiac troponin C (cTnC), from rainbow trout (Oncorhynchus mykiss), is a good model for studying temperature effects, both because of its low physiological temperature and because mammalian cTnC, extensively studied at higher temperatures, can be used for comparison. We determined the structure and studied the backbone dynamics of the regulatory domain of trout cardiac troponin C (ScNTnC) with one Ca2+ bound at 7 and 30 °C, using nuclear magnetic resonance spectroscopy (NMR). The overall fold of the regulatory domain of trout cTnC at both temperatures is similar to the regulatory domain of mammalian (human, bovine, and porcine isoform) cTnC bound to one Ca2+. By comparing the trout structures at the two temperatures, we identify differences between the positions of the helices flanking the calcium binding loops, and the overall structure at 7 °C is more compact than that at 30 °C. The structure at 7 °C is more similar to the mammalian cTnC, which was determined at 30 °C, indicating that they have the same conformation at their respective physiological temperatures. The dynamic properties of the regulatory domain of trout cTnC are similar at the two temperatures that were used in these studies.