Abstract We have measured the change in RNA transcript diversity during in vivo myogenesis in the chick limb by means of saturation hybridization of whole cell RNA to nonrepetitive DNA. We find that transcript diversity decreases as proliferating myoblasts fuse to form multinucleate myotubes. Specifically, RNA from limb mesoblasts at, or about, the time of phenotypic commitment (stage 24, Day 4.5) hybridizes to twice as much nonrepetitive DNA as does RNA from fully fused muscle (Day 18 of development through adulthood). The level of transcript diversity found at stage 24 remains high throughout the proliferative phase of myogenesis (through Day 12) and diminishes during the fusion phase (Days 10–18). Additive hybridization experiments indicate that the RNA present during fusion is homologous to RNA present in proliferating myoblasts, suggesting that the change in transcript diversity does not include large scale transcription of previously untranscribed sequences during this in vivo developmental transition.