The role of cardiac myosin binding protein-C (cMyBP-C) phosphorylation in cardiac physiology or pathophysiology is unclear. To investigate the status of cMyBP-C phosphorylation in vivo, we determined its phosphorylation state in stressed and unstressed mouse hearts. cMyBP-C phosphorylation is significantly decreased during the development of heart failure or pathologic hypertrophy. We then generated transgenic (TG) mice in which the phosphorylation sites of cMyBP-C were changed to nonphosphorylatable alanines (MyBP-C ^AllP− ). A TG line showing &40% replacement with MyBP-C ^AllP− showed no changes in morbidity or mortality but displayed depressed cardiac contractility, altered sarcomeric structure and upregulation of transcripts associated with a hypertrophic response. To explore the effect of complete replacement of endogenous cMyBP-C with MyBP-C ^AllP− , the mice were bred into the MyBP-C ^(t/t) background, in which less than 10% of normal levels of a truncated MyBP-C are present. Although MyBP-C ^AllP− was incorporated into the sarcomere and expressed at normal levels, the mutant protein could not rescue the MyBP-C ^(t/t) phenotype. The mice developed significant cardiac hypertrophy with myofibrillar disarray and fibrosis, similar to what was observed in the MyBP-C ^(t/t) animals. In contrast, when the MyBP-C ^(t/t) mice were bred to a TG line expressing normal MyBP-C (MyBP-C ^WT ), the MyBP-C ^(t/t) phenotype was rescued. These data suggest that cMyBP-C phosphorylation is essential for normal cardiac function.