Although Drosophila is widely used as a model for genetic diseases, few studies have reported ultrastructural analyses of cardiac muscle in heart disease models. To investigate ultrastructural defects associated with heart disease, transmission electron microscopy was performed on thin sections of striated muscles of Drosophila models for restrictive cardiomyopathy (RCM) and tau-induced cardiomyopathy. Although molecular causes of RCM are unknown, they can be associated with defects in myosin, a motor protein that generates contractile force in striated muscle. A single S1/S2 junction region mutation (P838L) in human cardiac _-myosin heavy chain was reported in a patient with RCM. To explore the efficacy of using Drosophila as a model for myosin-induced RCM, we hypothesized that the P838L myosin mutation in Drosophila would lead to myofibrillar abnormalities. We observed a progressive skeletal muscle ultrastructural phenotype in P838L myosin mutants (in myosin null Mhc_0 background), which includes myofibrillar disorganization and degradation. In addition, P838L-6F mutant indirect flight muscle (IFM) sarcomeres were hypercontracted compared to PWMhc2 controls. Our results suggest that the S1/S2 junction of myosin is important for regulating myosin structure and function. Mutations in this region may lead to alterations in the orientation of myosin heads relative to the cross bridge, leading to hypercontractility of sarcomeres and/or decreases in stiffness of the molecule. In human cardiac tissue, disruptions in stiffness may be associated with impairments in ventricular relaxation, leading to diastolic dysfunction. However, in Drosophila, the cardiac ultrastructure was normal in P838L-6F mutants compared to PWMhc2 controls in Mhc_ myosin null hearts. Thus, some cardiac phenotypes in the human condition may not translate to the Drosophila system. Severe cardiac phenotypes were observed in Drosophila hearts expressing mutant human tau. Hyperphosphorylation and aggregation of mutant tau into tangles occurs in several neurodegenerative diseases referred to as tauopathies. Accumulation of phosphorylated tau in cardiac and skeletal muscle biopsy samples suggest that striated muscle tissue generates tau-amyloid which causes the destruction/malfunctioning of myocytes. However, it is unknown how mutations in the tau gene leads to myopathies and moreover, there is no experimental model to understand tau-mediated striated muscle dysfunction. To test the effects of tau mutations on cardiac structure and function, we developed a novel Drosophila model that expresses pathological human tau in the heart using the UAS-Gal4 expression system with a cardiac specific driver. We first studied the effects of expressing mutant human tau (h-tauR406W and h-tauV337M) in IFMs using the Act88F driver and found that it resulted in reduced flight ability and ultrastructural defects, including myofibrillar disorganization and mitochondrial abnormalities. We also observed cardiac functional defects and progressive mitochondrial and myofibrillar ultrastructural abnormalities in the same tau mutants. Interestingly, these defects were suppressed by cardiac overexpression of DRP1 (a regulator of mitochondrial biogenesis), or TRAP1 (a mitochondrial chaperone) in 4 week-old tau mutant hearts. Our data demonstrate pathological consequences for tau mutations in striated muscle and a link between tau-induced myopathies and mitochondrial defects. These studies implement a novel approach to studying ultrastructural defects associated with cardiomyopathies using Drosophila as a model system.