Oviposition site selection by Tenebrio molitor L. was studied in cylindrical cages 10 cm in diameter × 28 cm in height using females up to 16 days old. Oviposition site selection is not a haphazard process. The adults are surface dwellers and the females normally enter the foodstuff to oviposit. The distance travelled into the foodstuff was age-related; the females went gradually deeper until the 9th day after eclosion. Physical and chemical factors (i.e. type and quality of foodstuff) and biological factors (i.e. adult density and presence of T. molitor eggs and larvae) were assessed when oviposition sites were selected by this insect. The females attempted to avoid poor quality food when selecting oviposition sites and usually laid fewer eggs and died sooner when poor quality food was present. As adult density increased, the females went deeper to oviposit. The females also went deeper to oviposit when eggs and larvae were present, but the effects of eggs and larvae were small and there seemed to be threshold densities as far as both factors were concerned. The behavioural significance of the data is discussed.