The amino acid composition of the developing larval cuticle and puparium of Calliphora vomitoria has been examined by means of paper chromatography. The most significant changes during development concern the aromatic amino acids. Phenylalanine and para-tyrosine apparently combine with the tanning quinone when the puparium hardens, and ortho-tyrosine is present for a brief period at about 7 days The presence of o-tyrosine, together with evidence resulting from the extraction of larval cuticles with water, gives rise to the suspicion that aromatic amino acids may be metabolized in the cuticle during development and give rise to hydroxylated products. The cuticle of 'permanent larvae', which fail to pupate even after the lapse of 2 to 3 weeks owing to the extirpation of Weismann's ring before the end of the normal feeding period, has also been examined. Its amino acid composition shows none of the changes seen in the normal larva.