Abstract 1. 1. The fat-body soluble fraction from two-day Sarcophaga bullata larvae contain amino acid activating enzymes for nineteen amino acids. 2. 2. The level of activity varies with the amino acid substrate. 3. 3. The total 32PP-ATP exchange activity of the pupae decreased with age for the first 6 days, then increased to a maximum one or two days prior to emergence of the adults. 4. 4. The free amino acid concentration in the pupae decreased during the period when the amino acid activating activity increased.