Variations of birth intervals within families were studied for a population living in the village of Crulai (in Normandy), including 46 couples married in the years from 1674 to 1742. Completed families with at least 6 children were studied. Comparisons were confined to intervals where the preceding child had attained at least one year of age, thus eliminating the variation in birth interval owing to infant mortality. Although previous studies of populations practicing no birth control show the effect of birth order (or of mother''s age) to be negligible, among the families of Crulai, intervals between successive confinements tend to increase with the order of birth. Further, some families were found to bear children within shorter intervals than other families. Intervals* between confinements depend on 3 factors: (1) the woman''s fecundability; (2) the frequency of uninduced miscarriages and still-births; (3) the time-lapse between a confinement and the next onset of ovulation. The data obtained favor the 3d factor since a constant shape of the successive birth interval curves occurs. Either birth order or mother''s age has an effect on the interval between births, there apparently being an increasing delay in the resumption of ovulation after a confinement. There is a marked variation in individual procreative ability as shown by inter-family differences.