Leishmania donovani strain 3S (Malakal area, Sudan) was adapted gradually to growing at temperatures that ranged from 30° to 37°C. Rich cultures of the parasites could be maintained by serial transfers at the various temperatures in modified Tobie's medium. Growth of L. donovani at temperatures exceeding 32°C occurred only in media supplemented with chick-embryo extract. At all temperatures, cultures of fully-adapted, vigorous organisms consisted primarily of typical promastigote stages. Flagellates grown at the several temperatures retained infectivity for hamsters as well as for cultures of L, human heart, human liver, and HeLa cell lines.