The criteria established by Drach for subdividing the molting cycle into stages are reviewed, and a suggestion is made for improving the uniformity of postmolt staging of different species. Changes in the epidermis during the molting cycle of the crayfish Orconectes obscurus and O. sanborni are described. Epidermal DNA content was measured throughout the cycle and found to drop sharply at stage D0 and to rise sharply at stage A. Protein content declined during postmolt and rose during premolt, as expected. Protein synthesis remained more or less constant during postmolt, rose during premolt, and dropped to the postmolt level at ecdysis. Chitin synthesis appeared to follow two different curves depending upon whether labelled glucose or acetylglucosamine was used as precursor. This, and the presence of a separate enzyme capable of phosphorylating acetylglucosamine and not glucose, suggests that acetylglucosamine may be utilized directly without prior conversion to glucose. Actinomycin D was found to prevent increases in rate of chitin biosynthesis during premolt but not to inhibit chitin biosynthesis already underway. During the same period, actinomycin stimulated general protein biosynthesis. By utilizing the molt staging criteria described, we were able to detect induction of premolt by ecdysone.