PRACTICE, with feedback, is a fundamental variable that influences the aquisition of motor skills(1) : with it, everyone improves, but some improve more than others, This simple fact has led to frequent debate over the relative importance of genetic and environmental influences on motor learning, In principle these factors could influence subjects’ initial level of proficiency, their rate of improvement or their final level of attainment, The problem has been investigated using the rotary pursuit (RP) task, in which subjects learn to track a rotating target with a stylus(2); this is a factorially pure task which is relatively unaffected by cognitive or verbal factors(3,4). Earlier studies of twins reared together(2,5) indicated that heredity was the primary factor responsible for individual differences in motor skill, Here we have studied learning in a sample of monozygotic (MZA) and dizygotic (DZA) twins who had been reared apart, Heritability of performance mas high even in the initial phase, and increased with practice, The rate of learning was also significantly heritable, We propose that the effect of practice is to decrease the effect of environmental variation (previous learning) and increase the relative strength of genetic influences on motor performance.