The formation of new myofibers in vertebrates occurs by myoblast fusion and requires fusogenic activity of the muscle-specific membrane protein myomaker. Here， using (BLAST) genome analyses， we show that the gene from trout includes 14 minisatellites， indicating that it has an unusual structure compared with those of other animal species. We found that the trout gene encodes a 434-amino acid (aa) protein， in accordance with its apparent molecular mass (∼40 kDa) observed by immunoblotting. The first half of the trout myomaker protein (1-220 aa) is similar to the 221-aa mouse myomaker protein， whereas the second half (222-234 aa) does not correspond to any known motifs and arises from two protein extensions. The first extension (∼70 aa) apparently appeared with the radiation of the bony fish clade Euteleostei， whereas the second extension (up to 236 aa) is restricted to the superorder Protacanthopterygii (containing salmonids and pike) and corresponds to the insertion of minisatellites having a length of 30 nucleotides. According to gene expression analyses， trout expression is consistently associated with the formation of new myofibers during embryonic development， postlarval growth， and muscle regeneration. Using cell-mixing experiments， we observed that trout myomaker has retained the ability to drive the fusion of mouse fibroblasts with C2C12 myoblasts. Our work reveals that trout myomaker has fusogenic function despite containing two protein extensions.