1. The common vampire bat,Desmodus rotundus, is known to be capable of detecting temperature differences. In the experiments described here two animals were trained to give preference to the radiation emitted by a warm signal unit (the negative unit was left at room temperature). The distance between the decision line and the signal units was 8, 12 or 16 cm. Both animals responded to radiation as low as 0.5 × 10−4 W cm−2 (Fig. 3). 2. Based on calculations of the radiation emitted by the human skin, vampires should be capable of detecting warm-blooded animals at a distance up to 16 cm (Fig. 4). This sense for thermal radiation may aid these bats in localizing or selecting a suitable biting site. 3. Histological studies of the bats' facial structures indicate that thermal stimuli are most probably perceived in the three pits surrounding the central nose leaf: the thin, hairless and glandless skin is underlaid with dense connective tissue. Thermography reveals that the surface temperature of the nasal region is up to 9 °C lower than that of the neighboring parts of the face (Fig. 2).