In striated muscle the structural genes associated with muscle fiber phenotype determination as well as muscle mass accretion are regulated largely by mechanical stimuli. Passive stretch of skeletal muscle stimulates muscle growth/hypertrophy and an increased expression of slow muscle genes. We previously identified Ankyrin repeat domain protein (Ankrd2) as a novel transcript expressed in fast tibialis anterior (TA) muscles after 7 days of passive stretch immobilization in vivo. Here we test the hypothesis that the expression of nkrd2 in stretched fast muscle is associated with the stretched-induced expression of slow muscle phenotype rather than the hypertrophic response. Our results show that in 4 and 7 days stretched TA muscle the expression of Ankrd2 mRNA and protein was significantly upregulated (p> 0.001). However in fast muscles of kyphoscoliotic mutant mice, which lack the hypertrophic response to overload but have a slower muscle phenotype than their wildtype littermate, Ankrd2 expression was significantly upregulated. The distribution pattern of Ankrd2 in fast and slow muscle is also in accord with their slow fiber composition. Furthermore it was markedly downregulated in denervated rat soleus muscle, which produces a pronounced shift towards the fast muscle phenotype. Using a sensitive proteomics approach (Ciphergen technology) we observed that Ankrd2 protein was undetectable in soleus after 4 weeks of denervation. We suggest that Ankrd2, which is also a titin binding protein, is a stretch response gene associated with slow muscle function and that it is part of a separate mechanotransduction system to the one which regulates muscle mass.