1. Studies on cyclic activity of thyroid in relation to environmental factors were made in an air breathing siluroid fish, H. fossilis. 2. The activity of thyroid gland showed parallelism with water temperature and photoperiod but from the regression analysis it was evident that the effect of photoperiod on thyroid activity is more pronounced than the effect of water temperature. 3. The maximum thyroid activity coincides well with the breeding phase of this animal.