To determine if a circadian response to thyroxine (T4) is exhibited byXenopus laevistadpoles, an experiment was performed in which groups of tadpoles were maintained in thyroxine‐treated or normal water on a 12‐hr photoperiod with a water temperature of approximately 20° C. All the groups were fed daily at the onset of the photoperiod. The thyroxine‐treated tadpoles were in thyroxine‐treated water either during the light or the dark phase of the photoperiod. These tadpoles were moved into and out of the thyroxine‐treated water every 12 hrs. Similarly, control groups were moved into and out of normal water every 12 hrs. Other control groups were maintained in normal water which was changed every 48 hrs. Using tail reabsorption as an index of metamorphosis, it was found that the mean length of the tadpoles was significantly shorter in tadpoles which were exposed to thyroxine during the light phase. According to theNormal Table of Xenopus laevis(Nieuwkoop and Faber, 1975), the tadpoles exposed to thyroxine during the light phase also showed the greatest change in morphology such as limb development.