Abstract We have previously observed that the cytosolic sialidase gene is highly expressed in rat skeletal muscle and that it contains an enhancer/promotor region which is transcriptionally active in rat L6 myogenic cells. Here we present evidence for the involvement of cytosolic sialidase in myoblast differentiation. During L6 myoblast differentiation induced by serum depletion, cytosolic sialidase is increased in activity as well as in terms of mRNA level. Sialidase activity is essentially lacking in untreated myoblasts but appears concomitantly with myotube formation after induction of differentiation. The mRNA becomes able to be detected after 3 days at the time which myogenin mRNA reaches a maximum level. Myotube formation can, in fact, be blocked bythe addition of an antisense oligodeoxyribonucleotide complementary to the first 8 codons of the cytosolic sialidase.