Myosin18a is a unique unconventional myosin with a PDZ domain.Although the gene is evolutionarily conserved from Drosophila to vertebrates, its expression and cellular function in various species remain unclear.We show that there are two myosin18a-like genes (myosin18L1 and myosin18L2) in zebrafish and both are predominantly and transitionally expressed in somite boundary between 24 and 36 hpf.Knockdown of either myosin18L1 or myosin18L2 by morpholino approach has no effect on muscle cell specification but disrupts muscle fibre stability and integrity, resulting in muscle fibre lesions and embryos with locomotion defects.