Moulting patterns in the silkworm,Bomhyx mori L. (PM × NB4D2) under different photoperiodic conditions were studied. Moulting phenomenon, inBombyx mori seems to be under the control of the circadian phenomenon. The peaks for the second moult were observed during the dark phase of the photoperiodic schedule expressing nocturnal predominancy. Consequently these peaks for consecutive moults moved into the light part of the 24 h cycle. This clearly demonstrates the instar dependency of the moulting expression. Under LL condition, a damp-out situation in moulting was observed. Moulting durations were greatly affected by photoperiods. Shortest moulting durations were recorded for LD 11:13.