When yeast strains were cultivated on maltose, the synthesis of trehalose already started in the exponential phase of growth, well before exhaustion of the sugar from the medium. This active pattern of trehalose accumulation was also observed in a maltose constitutive mutant strain growing on glucose. However, this accumulation was completely prevented by deletion of the TPS1 gene coding for the catalytic subunit of the UDPglucose-linked trehalose-6-phosphate synthase, indicating that no alternative pathway for trehalose synthesis exists in yeast. The active pattern of trehalose accumulation seems to be consistent with the finding that trehalose-6-phosphate synthase is more active in strains growing on maltose than on glucose.