In ectotherms, compensatory changes in ion channel number and activity are needed to maintain proper cardiac function at variable temperatures. The rapid component of the delayed rectifier K⁺ current, I_Kr, is important for repolarization of cardiac action potential (AP), and therefore crucial for regulation of cellular excitability and heart rate. To examine temperature plasticity of cardiac I_Kr, we cloned the ERG channel and measured its electrophysiological properties in thermally acclimated rainbow trout (Oncorhynchus mykiss). The present findings demonstrate a complete thermal compensation in the whole-cell conductance of the atrial I_Kr in rainbow trout acclimated to 4°C (CA, cold-acclimation) and 18°C (WA, warm-acclimation). In situ hybridization indicates that transcripts of the trout ERG channel (omERG) are present throughout the muscular tissue of the heart, and quantitative PCR shows increased expression of the omERG in CA trout in comparison to WA trout. In both acclimation groups, omERG expression is higher in atrium than ventricle. In addition, the omERG has some functional features that support I_Kr activity at low temperatures. Voltage dependence of steady-state activation is completely resistant to temperature changes, and steady-state inactivation and activation kinetics are little affected by temperatures below 11°C. Collectively, these findings suggest that high density of cardiac I_Kr is achieved by cold-induced increase in the number of functional omERG channels and inherent insensitivity of the omERG to temperature below 11°C. These adaptations are probably important in maintaining high heart rates and proper excitability and contractility of trout cardiac myocytes in the cold.