MyoD is a muscle-specific transcription factor involved in commitment of cells to myogenesis. MyoD mRNA levels differ between fast and slow muscles, suggesting that MyoD may regulate aspects of fibre type. Here we show that detectable MyoD protein becomes restricted during development to the nuclei of the fastest classes of fibres in fast muscles. myoDm1 mice, in which the myoD gene has been disrupted, show subtle shifts in fibre type of fast muscles toward a slower character, suggesting that MyoD is involved in the maintenance of the fast IIB/IIX fibre type. In contrast, slow muscle shifts to a faster phenotype in myoDm1. Moreover, MD6.0-lacZ transgenic mice with the myoD promoter driving lacZ, show highest beta-galactosidase activity in the fastest fibres of fast muscles, but also express low levels in slow fibres of slow, but not fast, muscles, suggesting distinct regulation of gene expression in slow fibres of fast and slow muscles.