Abstract 1. 1. The concentration of free and bound ascorbic acid, and of dehydroascorbic acid, is reported for five insect species: Musca domestica, Cutelia sedilotti, Dasycolletes hirtipes (Hudson), Melampsalata, cingulata, and Apis mellifera. 2. 2. The tissue distribution of reducing compounds is described. 3. 3. Bound hydroxyproline was found in all five species, but free hydroxyproline was detected only in Apis and Dasycolettes. 4. 4. The oxidation of l-tyrosine by Cutelia homogenates was activated by ascorbic acid and the other cofactor requirements of this system were determined. 5. 5. Ascorbate oxidation by homogenates of the two Hymenoptera occurred best at pH 5·5. 6. 6. Musca homogenates were shown to synthesize ascorbic acid from a variety of hexoses.