The gene for trout cardiac troponin I (ScTnI) has been cloned from trout cardiac cDNA and the protein expressed. ScTnI is lacking the 32 residue N-terminal overhang characteristic of mammalian cTnI, but absent in mammalian skeletal TnI, This overhang contains two serines that are phosphorylated by PKA in response to [beta]-adrenergic stimulation and are important in the regulation of cardiac function in mammals. This suggests that regulation of ScTn may be less complex than in mammalian cTn, and is perhaps more skeletal-like. The contribution of ScTnI to the Ca+2 binding properties of the troponin complex were explored using fluorescently-labelled mammalian cTnC. Ca+2 concentrations at half maximal fluorescence (pCa 50) suggest that the proteins did not complex properly in the Ca+2 titration buffer. Ca+2 off-rate ('k'off) values suggest that ScTnI does not affect the off-rate of Ca+2 for McTnC in complex. It is possible, however, that ScTnI instead affects the Ca+2 on-rate (' k'on). Additionally, ScTnI may need to be complexed with ScTnT and/or ScTnC to exert its influence on complex function.