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1. 1. A study has been made of the physiological relationships between β-alanine, tyrosine and a naturally occurring dipeptide, β-alanyl- l -tyrosine (sarcophagine), in the fly Sarcophaga bullata . 2. 2. The rate of sarcophagine biosynthesis in the growing larva is about 0·03 μmole/larva per hr, and this accounts for some 40 per cent of the total free β-alanine turned over. In the fully grown larva the dipeptide is the principal non-protein, ninhydrin-positive material, and it sequesters up to 90 per cent of the non-protein β-alanine and tyrosine. 3. 3. During hardening and darkening of the fly puparium, the concentration of sarcophagine and free tyrosine drops precipitously. The observed decrease in peptide-linked and free β-alanine at this time is accounted for quantitatively by the β-alanine bound in the sclerotized puparium. None of these changes occurs in the posterior portions of larvae ligatured behind the brain to prevent pupation. 4. 4. The question of whether the intact dipeptide or its individual hydrolysis products are incorporated into the sclerotized puparium is discussed; the bulk of the evidence suggests that hydrolysis precedes utilization.

The larvae described in this paper were obtained in 1916 by Major C. E. Southon, I.M.S., from a case of intestinal myiasis observed by him in Nairobi (British East Africa). These larvae were sent to Professor J. H. Ashworth and referred to me by Dr G. A. K. Marshall of the Imperial Bureau of Entomology.