The technique of path analysis was used to assess inherited and environmental variance components in physical fitness indicators measured in 1630 subjects from 375 families of French descent living in the greater Québec city area. For that purpose, submaximal power output (PWC150/kg), muscular endurance, muscular strength, reaction time and movement time were evaluated during a visit of the family to the laboratory. Inter-class correlations in various types of relatives were computed from scores adjusted for linear and non-linear effects of age and sex by a regression procedure (Y = age + sex + (age X sex) + age2). Correlations were then used in the path analytic BETA model which allows the partition of transmissible variance (t2) into genetic (h2) and cultural (b2) components. Results indicated that t2 accounted for 18% (movement time) to 63% (muscular strength) of the phenotypic variance. The contribution of genetic factors was found to be negligible for PWC150/kg and movement time, and accounted for about 20% of the phenotypic variance for reaction time and muscular endurance and 30% for muscular strength, while non-transmissible variance (1 - t2) accounted for 37% (muscular strength) to 82% (movement time) of the phenotypic variance. These results suggest that biological variation observed in the physical fitness level of a healthy population is mainly associated with non-transmissible environment factors and that the contribution of heredity is moderate and clearly lower than previously reported.