Sixty-five pairs of two-egg cattle twins of three breeds, Ayrshire, Friesian, and Shorthorn, were split at random within a week of their birth between two groups of commercial dairy farms. One group was selected for a relatively high average milk yield of the adult dairy herd (group A) and the other for relatively low average yield (group B). The body growth of the twins up to nearly 4 years old is described in this paper. Very significant differences in growth developed in consequence of the split between the groups. Twins on the A farms became larger than their co-twins on the B farms. Average differences between the groups increased for the first year or 18 months and then began to fall away, but had not disappeared at the last age (198 weeks) studied. The various body parts behaved differently i n response to the treatments or environments associated with the two groups of farms. Thus, at the ages studied, the twins on the ‘high’ yield farms differed in conformation from their co-twins on the ‘low’ yield farms. In addition to the differences arising between the members of split twin pairs, there were also differences in body size between the pairs. Breed was not apparently important as a factor affecting the growth differences arising between the two treatments.