Cholesterol content of Bombyx mori larvae was measured in two different silkworm groups, normally spinning and nonspinning, respectively. Cholesterol content in the spinning group showed very high values immediately after the moults, following which the concentration decreased and reached a minimum before the next moult. Alternatively, cholesterol content in the nonspinning group during the fifth instar increased up to the death of the larvae. Comparable behaviour was observed in a small percentage (about 20%) of nonspinning larvae of the second group. The reason for the nonspinning behaviour is not clear.