Galectin‐1 has been implicated in the development of skeletal muscle, being maximally expressed at the time of myofiber formation. Furthermore, in the presence of exogenous galectin‐1, mononuclear myoblasts show increased fusion in vitro. In the current study, we have used the galectin‐1 null mouse to elucidate the role of galectin‐1 in skeletal muscle development and regeneration. Myoblasts derived from the galectin‐1 mutant showed a reduced ability to fuse in vitro. In galectin‐1 null mutants, there was evidence of a delay in muscle fiber development at the neonatal stage and muscle fiber diameter was reduced when compared with wild‐type at the adult stage. Muscle regeneration was also compromised in the galectin‐1 mutant with the process being delayed and a reduced fiber size being maintained. These results, therefore, show a definitive role for galectin‐1 in fusion of myoblasts both in vitro, in vivo, and in regeneration after recovery from induced injury. Developmental Dynamics 236:1014–1024, 2007. © 2007 Wiley‐Liss, Inc.