Kyphoscoliotic (ky) mice are spontaneous mutants of the BDL strain whose postural muscles atrophy during post-natal growth, resulting in extensive kyphoscoliosis in adult animals. At 21 days of age, the seven muscles examined were already well differentiated into fast, slow and mixed type on the basis of the proportions of their native myosin isoforms or their subunits. During post-natal growth, from 21 to 120 days of age, the normal pattern of myosin maturation was essentially respected by the ky mutation: fast muscles became faster, slow muscles became slower and mixed muscles specialized in both directions. However, the post-natal increases of myosin heavy chain 2B and fast myosin light chain LC3f were depressed in ky muscles, whilst there was novel expression of slow myosin light chains, LC1s and LC2s in muscles which normally did not express them. Intermediate native myosin IM was absent in adult ky soleus, but it increased in adult ky tibialis anterior. We conclude that the ky mutation depresses the normal post-natal transition towards faster muscles and results in adult muscles whose myosin isoforms are generally shifted in a fast-to-slow direction.