Laboratory experiments conducted at L: D = 16: 8 have shown that the observed temporal pattern of parasitization of the Angoumois grain moth, Sitotroga cerealella eggs by Trichogramma principium females represents a resultant of arrhythmic age-related trends and circadian rhythms. Most of females delayed parasitization. The daily number of females starting to parasitize was maximal on the first day of contact with the host and then gradually declined. Practically all of the females started parasitization during the photophase. Moreover, when the first contact with the host fell on the scotophase, the total cumulative percentage of females that started parasitization during four days of the experiment significantly decreased. Oviposition activities of parasitizing females also occurred mainly during photophase. However, under constant light, these circadian rhythms were damped out after one cycle. In addition, anticipatory period of darkness during photophase directly inhibited parasitization. This suggests that the observed rhythms can be easily modified by the direct environmental influence. Under natural conditions, such a flexible oviposition rhythm may be of advantage for these parasitoids enabling them to use any opportunity for reproduction. In biocontrol practice, the lability of parasitization rhythms may enable Trichogramma females to adapt immediately to any new light-dark regimes, although darkness may have negative effects on their efficiency.