Abstract 1. 1. The free (physiological) amino acid (FAA) composition of the brain of minor workers of the red imported fire ant (RIFA), Solenopsis invicta, was conducted as well as an analysis of brain arginine concentrations in various worker subcastes. 2. 2. Nitrogenous waste products (urea and ammonia) together comprised 33% of the total ninhydrin-positive compounds. 3. 3. GABA, taurine, aspartate, glutamate and glycine together constituted 24% of the total ninhydrin-positive compounds. 4. 4. The most abundant FAA were alanine (16%), proline (12%), aspartate (7%) and glutamate (6%). 5. 5. No significant differences in brain arginine concentrations were found within a given head size class, within a given worker subcaste or between various subcastes, suggesting that arginine does not represent a neurochemical correlate with respect to the developmental switch in the behavioral repetoire shown by S. invicta worker subcastes. 6. 6. Brain arginine concentrations reported for S. invicta are compared with those reported for other arthropods.