DWARF calves have occurred fairly frequently in many Hereford herds in recent years. In the purebred herd under study the defect first appeared when an attempt was made to line-breed to a particularly good sire, RM. This bull produced a dwarf calf out of a cow not directly related to other ♀♀ in the herd. Both parents had a common ♂ ancestor, Prince Domino, 4-5 generations removed. Further dwarf calves were sired by 2 sons of BM and by 2 other linebred Prince Domino bulls when mated to daughters of BM. When mated to unrelated ♀♀ they sired only normal calves. The defect is present at birth but becomes accentuated with age. Most of the affected calves die before reaching maturity. Dwarf calves are slightly wider and blockier in build than normal calves. In several of the defective calves chronic bloat developed. Available data suggest that the abnormality is inherited as a simple, monofactorial, autosomal recessive.