The tail of the Alytes obstetricans tadpole, isolated at different stages (end of proclimax and climax), was studied in organ culture. The addition of thyroxine at a concentration of 5.10(-7) induces an involution slower than in vivo. Besides, this regression is comparatively slower in similar conditions, than in Xenopus laevis. This delay could be explained by the important volume and the very developed musculature of this anuran tail. Ultrastructural controls reveal the form of the muscles, the neural tube and the phagocytic cells in the tail metamorphising in vitro.