Yeasts are ubiquitous in terrestrial and aquatic microbiota, yet their ecological functionality remains relatively unexplored in comparison with other micro-organisms. This paper formulates and tests the novel hypothesis that heat produced by the sugar catabolism of yeast populations inhabiting floral nectar can increase the temperature of floral nectar and, more generally, modify the within-flower thermal microenvironment. Two field experiments were designed to test this hypothesis for the winter-blooming herb Helleborus foetidus (Ranunculaceae). In experiment 1, the effect of yeasts on the within-flower thermal environment was tested by excluding them from flowers, while in experiment 2 the test involved artificial inoculation of virgin flowers with yeasts. Nectary temperature (T(nect)), within-flower air temperature (T(flow)) and external air temperature (T(air)) were measured on experimental and control flowers in both experiments. Experimental exclusion of yeasts from the nectaries significantly reduced, and experimental addition of yeasts significantly increased, the temperature excess of nectaries (DeltaT(nect) = T(nect) - T(air)) and the air space inside flowers in relation to the air just outside the flowers. In non-experimental flowers exposed to natural pollinator visitation, DeltaT(nect) was linearly related to log yeast cell density in nectar, and reached +6 degrees C in nectaries with the densest yeast populations. The warming effect of nectar-dwelling yeasts documented in this study suggests novel ecological mechanisms potentially linking nectarivorous microbes with winter-blooming plants and their insect pollinators.