Abstract Intermoult endocuticle is laid down progressively in the 5th instar of Calpodes ethlius. The incorporation of cuticular precursors into the intermoult endocuticle has been followed autoradiographically. The uptake of general cuticular precursors in the intermoult period is fast enough to clear most of an unchased pulse of 14C-acetate from the haemocoele to the edge of the epidermis and into the cuticle in 2 hr. There are two patterns of incorporation, layered and diffuse. 3H-glucose, a precursor of chitin, is incorporated in discrete layers corresponding to the new cuticle synthesized during the period of incorporation and deposited at the edge of the epidermis. Tritiated amino acids, on the other hand, are incorporated not only in layers but also diffusely throughout the thickness of the cuticle. The cuticle stretches during the instar as the larva feeds and grows. In spite of this, electron microscopical observations on the thickness of the lamellae suggest that an increase in separation of the first formed lamellae takes place by the end of the instar, presumably as a result of the material added in the diffuse form of incorporation. These experiments suggest that the endocuticle grows both by the addition of lamellae at the epidermal surface and by intussusception.