ABSTRACT A simple method has been devised for producing tetraploid newts by means of a ‘heat’ treatment. The eggs of T. palmatus (helveticus) and T. vulgaris were at different stages, varying from 3 hours to hours after artificial insemination, exposed to warm water of 34° –38°C. for a duration of 10–12 minutes. A temperature treatment of 35 ·6° –36·0° C. proved to be the most successful for the production of tetraploid embryos and larvae. Treatment at the stage when the first cleavage furrow closes at the vegetative pole was found to be the only effective one. The eggs contain at this stage two interphase nuclei. The described treatment causes up to 24 per cent, of all treated eggs, or up to 93·7 per cent, of all analysed newts to develop into tetraploids. The probable cytological mechanism which leads to tetraploidy is discussed. Factors which seem to be of importance for the production of tetraploidy in other amphibian species, and some important problems which might be investigated with the help of this method, are mentioned.