The rate of growth of the larval cuticle of Sarcophaga and the chemical composition of its layers are fully described. The cuticle consists essentially of two fundamentally different layers. The outer layer--the epicuticle--is about $4\mu $ thick, and contains no chitin. It is basically composed of protein, having its isoelectric point at pH 5.1. At its surface a resistant lipo-protein complex forms a very thin membrane which may be separated intact from the remainder of the epicuticle which contains little or no lipoid. The epicuticle is therefore a double structure. The inner layer of the cuticle--the endocuticle--is much thicker than the epicuticle, and is a laminated chitin-protein complex of isoelectric point pH 3.5. Like the epicuticle it is a double structure. The outer zone of the endocuticle is secreted during early larval life, and is penetrated by pore canals which do not enter the epicuticle. The inner zone of the endocuticle is secreted later in larval life, and contains no pore canals. An exocuticle is absent from the larval cuticle, but the outer endocuticle is destined to form the exocuticle of the puparium, the inner endocuticle remaining unchanged. The pore canals at first contain cytoplasmic filaments, but these later become coiled and replaced by chitin.