The seasonality of births in the period 1871-1977 is studied in a rural north-western Spanish population. Based on a total sample of 11,695 birth registrations, temporal variation is analysed. For siblings, according to family reconstitution, the total family size, the legitimacy of the child, and birth order are considered. A coefficient of birth month dispersion is defined and estimated for each family. Intra-family variation is related to inter-family coefficients in order to determine whether the local seasonal pattern of births may be partly explained by family characteristics.The seasonality of births in the period 1971 through 1977 is studied in a rural north-western Spanish population. Based on a total sample of 11,695 birth registrations, temporal variation is analyzed. For siblings, according to family reconstitution, the total family size, the legitimacy of the child, and birth order are considered. A coefficient of birth month dispersion is defined and estimated for each family. Intra-family variation is related to inter-family coefficients in order to determine whether the local seasonal patterns of births may be partly explained by family characteristics. The facts that the seasonal pattern of births does not vary with sibship size, and that, regardless of number of children, births occur throughout the year following the population pattern, are not to be confused with the differences in season of birth within each family. Within families, birth order did not affect the seasonal pattern but small families showed greater scores than the larger ones. Thus, in large families births are less dispersed throughout the year, perhaps reflecting some tendency to family seasonal preference of birth. In any case, the influence of the familial factor must be slight, since differences were not found when comparing simulated and observed families.