Distributions, correlations and weighted least squares estimates of the components of variation in right-left asymmetry for individual finger ridge counts have been obtained from 221 pairs of twins and 80 pairs of opposite sex siblings. Asymmetry has been measured by two indices: signed right-left difference representing unidirectional asymmetry and absolute difference, representing ambidirectional asymmetry. The results indicate that both types of asymmetry are largely under environmental control, but with significant genetic components, particularly in males. The proportion of genetic variation in these measures of asymmetry varies somewhat between individual fingers.