Recently I have reported experiments which have led me to throw doubt on the universal applicability of the theory of the production of a moult-inhibiting hormone by the eyestalks of the decapod Crustacea (Carlisle, 1953). I suggested that the eyestalk moult-inhibiting hormone might be associated with the existence of a definite moulting season. In animals which possessed such a moulting season moultingwould then be inhibited outside this season, but within the season there might well be no such inhibition. The experiments reported here support a modified form of this hypothesis.