Potential molecular mechanisms regulating developmental plasticity to temperature were investigated in Atlantic salmon embryos (Salmo salar L.). Six orthologues of the four myogenic regulatory factors (MRFs: individually: smyf5, smyoD1a/1b/1c, smyoG and sMRF4), the master transcription factors regulating vertebrate myogenesis, were characterised at the mRNA/genomic level. In situ hybridisation was performed with specific cRNA probes to determine the expression patterns of each gene during embryonic myogenesis. To place the MRF data in the context of known muscle fibre differentiation events, the expression of slow myosin light chain-1 and Pax7 were also investigated. Adaxial myoblasts expressed smyoD1a prior to and during somitogenesis followed by smyoD1c (20-somite stage, ss), and sMRF4 (25-30 ss), before spreading laterally across the myotome, followed closely by the adaxial cells. Smyf5 was detected prior to somitogenesis, but not in the adaxial cells in contrast to other teleosts studied. The expression domains of smyf5, smyoD1b and smyoG were not confined to the s-smlc1 expression field, indicating a role in fast muscle myogenesis. From the end of segmentation, each MRF was expressed to a greater or lesser extent in zones of new muscle fibre production, the precursor cells for which probably originated from the Pax7 expressing cell layer external to the single layer of s-smlc1(+) fibres. SmyoD1a and smyoG showed similar expression patterns with respect to somite stage at three different temperatures investigated (2 degrees C, 5 degrees C and 8 degrees C) in spite of different rates of somite formation (one somite added each 5 h, 8 h and 15 h at 8 degrees C, 5 degrees C and 2 degrees C, respectively). In contrast, the expression of smyf5, sMRF4 and s-smlc1 was retarded with respect to somite stage at 2 degrees C compared to 8 degrees C, potentially resulting in heterochronies in downstream pathways influencing later muscle phenotype.