The simultaneous presence of two different trehalose-hydrolysing activities has been recognised in several fungal species. While these enzymes, known as acid and neutral trehalases, share a strict specificity for trehalose, they are nevertheless rather different in subcellular localisation and in several biochemical and regulatory properties. The function of these apparently redundant activities in the same cell was not completely understood until recently. Biochemical and genetic studies now suggest that these enzymes may have specialised and exclusive roles in fungal cells. It is thought that neutral trehalases mobilise cytosolic trehalose, under the control of developmental programs, chemical and nutrient signals, or stress responses. On the other hand, acid trehalases appear not to mobilise cytosolic trehalose, but to act as 'carbon scavenger' hydrolases enabling cells to utilise exogenous trehalose as a carbon source, under the control of carbon catabolic regulatory circuits. Although much needs to be learned about the molecular identity of trehalases, it seems that in fungi at least one class of acid trehalases evolved independently from the other trehalases.