1. The histochemistry of the cuticle in different stages of the moult cycle of Carcinus maenas is described.
2. The newly formed cuticle of a crab exposed at moulting shows evidence of phenolic tanning in the epicuticle.
3. The subsequent tanning of the pigment layer of the endocuticle is confined to a brief period immediately after moulting.
4. The polyphenol oxidase of the epicuticle involved in tanning disappears soon after moulting, but some time later an oxidase is again indicated, this time in the pigment layer. It appears to be secreted by the tegumental glands.
5. From the concurrent appearance of a polyphenol oxidase and tyrosine and the subsequent formation of dihydroxyphenols and melanin-like substances in the cuticle, it is suggested that the oxidation products of tyrosine are responsible for the pigmentation of th e cuticle.
6. Pigmentation of the cuticle is discussed in relation to phenolic tanning in Crusta-cea and Insecta.