This study focuses on the quantification of genetic and environmental sources of variation in physical fitness components in 105 10-yr-old twin pairs and their parents. Nine motor tests and six skinfold measures were administered. Motor tests can be divided into those that are performance-related: static strength, explosive strength, running speed, speed of limb movement, and balance; and those that are health-related: trunk strength, functional strength, maximum oxygen uptake, and flexibility. The significance and contribution of genetic and environmental factors to variation in physical fitness were tested with model fitting. Performance-related fitness characteristics were moderately to highly heritable. The heritability estimates were slightly higher for health-related fitness characteristics. For most variables a simple model including genetic and specific environmental factors fitted the observed phenotypic variance well. Common environmental factors explained a significant part of the variation in speed components and flexibility. Assortative mating was significant and positive for speed components, balance, trunk strength, and cardiorespiratory fitness, but negative for adiposity. Static strength, explosive strength, functional strength, and cardiorespiratory fitness showed evidence for reduced genetic transmission or dominance. The hypothesis that performance-related fitness characteristics are more determined by genetic factors than health-related fitness was not supported. At this prepubertal age, genetic factors have the predominant effect on fitness.