Remodeling of adherens junction, gap junction, and desmosomal proteins at the intercalated discs of cardiomyocytes in heart characterizes several animal models of cardiomyopathy, especially dilated cardiac myopathy (DCM). In this study, we show that the tight junction protein, claudin-5, is present in cardiac muscle and localizes to the lateral membranes of cardiomyocytes in normal mice. We further examined claudin-5 in utrophin/dystrophin-deficient (double knockout, dko) mice, a mouse model of muscular dystrophy with cardiomyopathy, and found that claudin-5 mRNA and protein levels are decreased in dko hearts as compared with normal. Intercalated disc cell junction proteins, and another tight junction protein, zonula occludens-1 (ZO-1), are not altered in the dko mouse. Ultrastructural data from dko hearts also shows that the lateral membranes of cardiomyocytes exhibit an abnormal wavy appearance. These data demonstrate that claudin-5 is specifically altered in dko hearts, suggesting that alterations of the lateral membranes of cardiomyocytes, rather than intercalated discs, are associated with cardiomyopathy in the dko mouse.