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Maximal aerobic power (MAP) exhibits considerable variation between individuals within a population. Among all causal sources contributing to variation, heredity is generally thought to exert a rather significant influence. Six hundred and seven subjects (9 to 52 years of age) from 160 families of French descent, living in the greater Quebec city area, have been measured for MAP and several related biological and cultural indicators. Subjects have been submitted to a multistage submaxi- mal ergocycle test. MAP has been estimated by regression of actual measurements of oxygen intake and heart rate (HR) at each work load to mean maximal HR. Age and sex of subjects accounted for more than 50% of the total variation in MAP. Anova procedures revealed the presence of significant familial concentrations from MAP scores adjusted for age, sex, sum of skinfolds, cigarette smoking, current energy expenditure, weekly participation in aerobic activities and socio-economic status. Inter-class correlation analysis indicated a significant spouse resemblance (r = .34), as well as a significant covariation between parents and their children (r = .19) and between children of same sibships (r = .33). These results suggest that heredity is contributing to the variation in MAP, but much less than was previously reported from twin studies.