Abstract 1. 1. Extracts of tails of Rana grylio froglets show seventeen times the β-glucuronidase activity per gram when compared with tadpoles of this species. 2. 2. Comparison of properties of partially purified enzyme preparations from tadpole and froglet tails resulted in no demonstrable differences in thermal sensitivity or heats of activation. 3. 3. Some slight differences in apparent Km and pH dependency were noted, but may be due to the presence of modifiers in the preparations. 4. 4. A time study of the thermal decay at 73·5°C showed two decays, suggesting the existence of two components in each extract, each comprising about 50 per cent of the total activity in tadpole and in froglet preparations.