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Myostatin is a negative regulator of muscle growth and mutations in its gene lead to muscular hypertrophy and reduced fat. In cattle, this is seen in ‘double muscled’ breeds. We have used marker-assisted introgression to introduce a murine myostatin mutation, MstnCmpt-dl1Abc [Compact (C)], into an inbred line of mice (DUHi) that had been selected on body weight and had exceptional growth. Compared with homozygous wild-type mice, homozygous (C/C) mice of this line were ~4–5% lighter, had ~7–8% shorter tails, substantially increased muscle weights (e.g. quadriceps muscle in males was 59% heavier) and an increased ‘dressing percentage’ (~49% vs 39%), an indicator of overall muscularity. The weights of several organs (e.g. liver, kidney, heart and digestive tract) were significantly reduced, by 12–20%. Myostatin deficiency also resulted in drastic reductions of total body fat and of various fat depots, total body fat proportion falling from ~17·5% in wild-type animals of both sexes to 9·5% and 11·6% in homozygous (C/C) females and males, respectively. Males with a deficiency in myostatin had higher gains in muscle traits than females. Additionally, there was a strong distortion of the segregation ratio on the DUHi background. Of 838 genotyped pups from inter se matings 29%, 63% and 8% were homozygous wild type (+/+), heterozygous (C/+) and homozygous (C/C), respectively, showing that MstnCmpt-dl1Abc has lower fitness on this background. This line, when congenic, will be a useful resource in gene expression studies and for finding modifying genes.