Studies of the auxin content of developing fig fruits revealed a sudden 30-fold increase in auxin concentration in the variety Calimyrna at the time of formation of the endosperm tissue. A similar and even larger rise in the parthenocarpic Mission variety was associated with the development of a parthenogenetic endosperm. By chromatographic analysis this auxin was shown to be different from those present at earlier and later stages of growth and to be chemically distinct from indoleacetic acid. Its function is thought to be the prevention of fruit drop. No evidence was found that the rate of fruit enlargement was in any way dependent on the auxin concentration.