In vertebrate striated muscle, myosin is composed of 2 heavy chains of about 200,000 daltons each and 4 light chains of about 20,000 daltons each. The light chains are of 2 distinct types: the phosphorylatable, regulatory, or MLC2 type, and the nonphosphorylatable, alkali, or MLC1 and MLC3 types (MYL1 according to the HGM symbols). The previously uncharacterized zebrafish alkali-like myosin light chain gene (myl1) was found to be expressed exclusively in cells in the fast-twitch pathway initiated at an early stage of fast fibre differentiation. Myl1 was expressed earlier, and in a more fibre type restricted manner, than any of the previously described and frequently used fast myosin light and heavy chain and troponin muscle markers mylz2, mylz3, tnni2, tnnt3a, fMyHC1.3.