The psychrophobic yeasts Torulopsis pintolopesii, Torulopsis bovina and Candida slooffii were studied aiming at determining some of the factors important for the specificity of yeastdwelling in certain hosts. These factors seem to be related in part to the nutritional requirements of the strains. Stimulatory or inhibitory factors extracted from the stomach of different animal species were unrelated to the specific fixation of certain yeasts in their hosts. We cannot however exclude that factors present in the more distal areas of the digestive tract are responsible for this specific fixation. Studies on the viability of yeasts in presence of animal sera suggest that serum factors may be equally important in determining the host-specificity of some psychrophobic yeasts.