The aim of the present study was to precise the origin of the particular muscle characteristics of double-muscled cattle by comparing muscle properties of Holstein and double-muscled Belgian Blue (BB) foetuses. Ten 100-day-old foetuses of each genotype were studied. The weight and length of foetuses and the length, weight and area of the Semitendinosus (ST) muscle were analysed. Contractile differentiation of the different fibre types was studied by immunohistochemistry using several monoclonal antibodies raised against different myosin heavy chain isoforms (MHC slow, fast, foetal) and by electrophoresis. Proliferation phase of myoblasts from each genotype was analysed in primary culture. On 100 days of foetal life, the foetuses of both genotypes did not show any significant differences in their weight and length. However, BB cattle already present muscle hypertrophy, which seems to originate from a higher myoblast proliferation observed in primary culture. The use of anti-MHC antibodies shows that ST muscle of BB contained a smaller proportion of primary fibres and a higher proportion of secondary fibres which will give principally fast fibres in adult muscle. Electrophoresis analysis confirms a lower proportion of slow MHC in ST of BB.