Myostatin (MSTN), a transforming growth factor beta superfamily member, is an essential factor for the growth and development of muscle mass. The protein functions as a negative regulator of muscle growth and is related to the so-called double-muscling phenotype in cattle, where a series of mutations renders the gene inactive. One particular breed of pigs, the Belgian Piétrain, also shows a heavily muscled phenotype. The similarity of muscular phenotypes between the double-muscled cattle and Piétrain pigs indicated that MSTN may be a candidate gene for muscular hypertrophy in pigs. In this study, we sequenced and analysed the complete MSTN gene from 45 pigs of five different breeds, including the heavily muscled Piétrain breed at one extreme and the Meishan and Wild boar breeds at the other extreme. In total, 7626 bp of the porcine MSTN gene were sequenced, including the 5' and 3' UTR. Fifteen polymorphic loci were found, three of which were located in the promoter region, five in intron 1 and seven in intron 2. Most mutations were found when comparing the obtained MSTN sequence with porcine MSTN sequences already published. However, one polymorphism located at position 447 of the porcine MSTN promoter had a very high allele frequency in the Piétrain pig breed and disrupted a putative myocyte enhancer factor 3 binding site. Real-time PCR using Sybr Green showed that this mutation was associated with expression levels of the MSTN gene in m. longissimus dorsi at an age of 4 weeks.