The effect of different temperatures (4 degrees C and 12 degrees C) on myogenic regulatory factors (MyoD and myogenin) and myosin heavy chain (MyHC) expression was investigated in rainbow trout (Onchrhynchus mykiss) during early development. MyoD is first switched on at stage 14 [about 5 somites are formed (1/2 epiboly)] while myogenin mRNA is expressed at stage 15 [around 15 somites are visible (2/3 epiboly)] at both temperatures. Subsequently (up to at least stage 20), the most caudal somites exhibit less myogenin mRNA at 4 degrees C compared to 12 degrees C. At the eyed stage (stage 23-24), both myogenin mRNA and protein are present in greater amounts throughout all myotomes at the lower temperature, with mRNA levels in warmer (12 degrees C) embryos at 83% for MyoD and 72% for myogenin of the levels seen in 4 degrees C embryos. Conversely, however, at this same stage, fast-MyHC mRNA and protein are more abundant in 12 degrees C than in 4 degrees C embryos. This indicates relatively advanced muscle differentiation at the warmer temperature. At hatching, myogenin-positive cells are concentrated within the myosepta at both temperatures and they are also sparsely distributed in the myotome at 4 degrees C, but not at 12 degrees C. MyoD, myogenin, and MyHC levels provide an indication of differentiation of muscle cells. These findings suggest that myogenic regulatory factor expression is delayed but prolonged by the lowering of temperature.