Work on Belgian Blue cattle revealed that an 11 base pair (bp) deletion within the bovine myostatin gene (GDF8) is associated with the double-muscled phenotype seen in this breed. Investigations focusing on other European breeds known to show double-muscling identified several mutations within the coding region of the gene associated with the double-muscled phenotype in different breeds. The number of mutations found suggest that myostatin is highly variable within beef cattle. Variations that alter the structure of the gene product such that the protein is inactivated are associated with the most pronounced form of double-muscling as seen in the Belgian Blue. However, other mutations may have a less extreme affect on muscle development. While overt double-muscling gives rise to a high incidence of dystocia (calving difficulty), it is possible that some variants may give enhanced muscling, but with limited calving problems. We describe sequence analysis of the myostatin gene in ten beef breeds commonly used in the UK and show that the 11-bp deletion responsible for double-muscling in the Belgian Blue is also present in the South Devon cattle population. Allele frequencies and haplotypes in the South Devon and a polymerase chain reaction (PCR) based test for the deletion are described. PCR amplification across the deleted region provides a quick and effective test with clear identification of heterozygous individuals. We discuss our results with regard to the effect of genotype on phenotype and differences observed between the Belgian Blue and the South Devon.