During a screening for novel microbial trehalose phosphorylase three Pichia strains were identified as producers of this particular enzyme that have not yet been described. To our knowledge, this is the first time that this enzyme activity has been shown in yeasts. Pichia fermentans formed trehalose phosphorylase when cultivated on a growth medium containing easily metabolizable sugars such as glucose. Addition of NaCl (0.4 M) to the medium increased the synthesis of the enzyme significantly. Production of trehalose phosphorylase was found to be growth-associated with a maximum of activity formed at the transition of the exponential to the stationary phase of growth. Trehalose phosphorylase catalyzes the phosphorolytic cleavage of trehalose, yielding glucose 1-phosphate (glucose-1-P) and glucose as products. In vitro the enzyme readily catalyzes the reverse reaction, the synthesis of trehalose from glucose and glucose-1-P. For this reaction, the enzyme of P, fermentans was found to utilize alpha-glucose-1-P preferentially. A partially purified enzyme preparation showed a pH optimum of 6.3 for the synthesis of trehalose. The enzyme was found to be rather unstable; it was easily inactivated by dilution unless Ca2+ or Mn2+ were added. This instability is presumably caused by dissociation of the enzyme. In contrast to other yeasts, P. fermentans rapidly degraded intracellularly accumulated trehalose when the carbon source in the medium was depleted. Trehalose phosphorylase seems to be a key enzyme in the degradative pathway of trehalose in P. fermentans. Additional enzymes in this catabolic pathway of trehalose include phosphoglucomutase, glucose-6-phosphate dehydrogenase, and gluconolactonase.