ABSTRACT.A study of the individual variability in the length of intermoult periods allows correlations to be established between reproductive and moulting cycles in adult females ofThermobia domestica(Packard) (Thysanura, Lepismatidae). By keeping the females without or with males and by changing the day of insemination, it is shown that the intermoult periods vary with the rate of ovarian maturation from the beginning of each stadium. Females with slow oocyte growth are never inseminated, even in the presence of males; they have short intermoults. Females with rapid oocyte growth can be inseminated and the timing of insemination regulates the length of the intermoults. It appears that the variability in duration of the intermoult only concerns the first part of the stadium (postecdysial period of the reproductive cycle=period of intense vitellogenesis), whereas the second part of the stadium (pre‐ecdysial period=previtellogenesis) has an almost constant length. The endocrine mechanisms involved are considered, taking into account the cyclic changes in hormonal levels already described in other paper