After injections of U-14C-phenylalanine into the haemolymph of female Glossina morsitans Westw. during late pregnancy, radioactivity was detected in the post-parturient female and its larval offspring in the injected material and in lipids, a range of non-essential amino acids and tyrosine. After intrahaemocoelic administration of U-14C-tyrosine, radio-activity was detected in the adult and larva in all these nutriments except phenylalanine. The level of radioactivity in the larva was higher than that remaining in the female parent, and in both radioactivity was greater in amino acids than in the lipid fraction. Radiometric analysis of the oocyte and the infra-uterine larva 24 h after injection of labelled phenylalanine or tyrosine on different days of the pregnancy cycle revealed patterns of radioactivity relative to their respective growth characteristics. The rate of phenylalanine utilisation by the pregnant female for nourishment of the in-utero third-instar larva was significantly higher than the rate of tyrosine utilisation. For both labelled materials, there was a significantly higher utilisation for energy during early than during late pregnancy, but at both physiological ages tyrosine was metabolised for energy to a relatively greater degree. Uric acid is the main nitrogenous waste product of aromatic amino acid metabolism in this insect, though a very low level of activity was also detected in the amino acid fractions of the excreta from females in early and late pregnancy.