--- - |2+ Abstract Aim Tieg1 is involved in multiple signaling pathways, human diseases, and is highly expressed in muscle where its functions are poorly understood. Methods We have utilized Tieg1 KO mice to identify novel and important roles for this transcription factor in regulating muscle ultrastructure, metabolism and mitochondrial functions in the soleus and EDL muscles. RNA sequencing, immunoblotting, TEM, MRI, NMR, histochemical and mitochondrial function assays were performed. Results Loss of Tieg1 expression resulted in altered sarcomere organization and a significant decrease in mitochondrial number. Histochemical analyses demonstrated an absence of SDH staining and a decrease in COX enzyme activity in KO soleus with similar, but diminished, effects in the EDL. Decreased complex I, COX and citrate synthase activities were detected in the soleus muscle of KO mice indicating altered mitochondrial function. Complex I activity was also diminished in KO EDL. Significant decreases in citrate synthase and respiratory chain complex activities were identified in KO soleus. 1H‐NMR spectra revealed no significant metabolic difference between WT and KO muscles. However, 31P spectra revealed a significant decrease in phosphocreatine and ATPg. Altered expression of 279 genes, many of which play roles in mitochondrial and muscle function, were identified in KO soleus muscle. Ultimately, all of these changes resulted in an exercise intolerance phenotype in Tieg1 KO mice. Conclusion Our findings have implicated novel roles for Tieg1 in muscle including regulation of gene expression, metabolic activity and organization of tissue ultrastructure. This muscle phenotype resembles diseases associated with exercise intolerance and myopathies of unknown consequence. - 'Acta Physiologica, Volume 0, Issue ja, -Not available-. '