Skeletal muscle fibre characteristics are key determinants of meat quality. Highfibre diameters and shifting towards higher white fibre proportions lead toincreasing R-values (degree of desamination of adenosine) andlactate-production, resulting in high incidences of pale, soft, exudative (PSE)meat and stress susceptibility in European and American pig breeds. Developmentof muscle fibres including their enzymes, is regulated by the MyoD-gene familytogether with transcription factors like FOS. We report on the associationsbetween the chromosomal region of FOS with skeletal muscle fibre and metabolismtraits. The BB genotype representing the European Pietrain breed had 10.9% morewhite fibres with fibre diameters decreased by 6.1%, with 3.9% higher R-valuesand 8.5% higher lactate levels than the AA genotype of the Chinese Meishan.Lactate levels and R-values per microm of fibre diameter were increased to 18.4and 11.6% in the BB genotype. The contrast between the two quantitative traitloci (QTL) alleles associated with a polymorphism in the FOS gene explained upto 5.13% of the total variance. A new TaiI-restriction fragment lengthpolymorphism (RFLP) connected to a Asn258/Ser mutation, located in atranscription activator region, was used to map FOS between markers S0115 andSw581 on SSC7. The QTLs for skeletal muscle fibre and metabolism traits havebeen mapped to the marker interval around FOS. The present data suggest thatvariability in FOS gene may underlie phenotypic variation in skeletal musclefibre and metabolism traits in the pig.