Trout cardiac myofibrils are ∼10-fold more sensitive to Ca+2 than those from mammalian hearts when measured at the same temperature. It has been demonstrated that trout cardiac troponin C (ScTnC) has 2.3 fold the Ca2+ affinity of human cTnC and is responsible for a 2-fold increase in cardiac myofibril Ca+2 sensitivity. The contributions of trout cardiac troponin I (ScTnI) to the Ca+2 sensitivity of the trout heart is currently unknown. The cDNA for ScTnI has been cloned using RACE-PCR. Sequencing results indicate that ScTnI is 59% and 56% identical to human cTnI and human skeletal troponin I, respectively, at the amino acid level. Interestingly, ScTnI lacks the ∼30-residue N-terminal sequence present in mammalian cTnI that contains two protein kinase A (PKA) target residues at positions 23 and 24. ScTnI has been expressed and the influence of it on the Ca2+ activation of human cardiac troponin is currently being characterized using steady state Ca2+ binding assays and stopped flow kinetic analysis. This work is supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Foundation for Innovation (CFI) to TEG.