The efficacy of D-glucose labeled with 13C6 (D-glucose-13C6) in the study of amino acid metabolism in insects was determined by incorporating either D-glucose-13C6 or unlabeled D-glucose into larval and adult diets of the Mediterranean fruit fly (medfly), Ceratitis capitata (Wiedemann) (Diptera: Tephritidae). Feeding treatments were: (a) labeled glucose diet for larvae + unlabeled glucose diet for adults; (b) unlabeled glucose diet for larvae + labeled glucose diet for adults; (c) labeled glucose diet for larvae + labeled glucose diet for adults; and (d) unlabeled glucose diet for larvae + unlabeled glucose diet for adults. Amino acid content of extracts from unmated adults and mated adults was determined with gas chromatography-mass spectrometry (GC-MS). L-alanine, L-aspartic acid, L-cysteine, L-glutamic acid, L-glycine, L-isoleucine, L-leucine, L-methionine, L-proline, L-phenylalanine, L-serine, L-threonine, L-tyrosine, and L-valine were identified from the insect extracts. In the treatment that fed unlabeled glucose larval and adult diets (control), amino acid content was lower after mating than before mating except for L-cysteine (e.g., 13-fold increase). Amino acid content in insects fed D-glucose-13C6 also decreased after mating except for L-valine, L-leucine, L-tyrosine, and L-isoleucine, all of which remained comparatively equal before and after mating, and L-threonine which increased almost 12-fold after mating. Only trace amounts of L-tyrosine and L-isoleucine were detected before and after mating in those fed labeled diets. These results suggest that stable isotopes can be used as labels for insect metabolic studies.