To grow on cellulose as a carbon source, Hypocrea jecorina (Trichoderma reesei) expresses and secretes a number of cellulases. This mechanism of induction by an insoluble carbon source has been controversially explained, but is most frequently attributed to the formation of the β-1,2-diglucoside sophorose, a powerful soluble inducer of cellulases, by means of transglycosylation by constitutive or conidia-bound β-glycoside hydrolases. Some recent results, however, have put the role of sophorose as the mediator of cellulose induction in question. Here we used the rapid subtraction hybridization approach to clone genes expressed by H. jecorina in the presence of cellulose but not upon incubation with sophorose. From a total of 96 expressed sequence tag (EST) fragments, 37 putative positives—representing ten different genes—were selected and analysed. All of them were present in the genome sequence of H. jecorina. Three of them encode proteins known from H. jecorina, five encode enzymes involved in secondary metabolism and one gene encodes an as yet unknown member of glycoside hydrolase family 30. Two EST fragments had no orthologues in other fungi. One of them made up for 25 of the 37 EST fragments analysed. The corresponding gene (only expressed on cellulose, ooc1) encodes a small secreted 10.5-kDa protein. The ooc1 transcript is only detectable during growth on cellulose in darkness, but not on cellulose in light or in the presence of other cellulase inducers (sophorose, lactose), nor is it formed during growth on glucose or glycerol. Its expression is strongly reduced, but not completely abolished in the cellulase non-inducible mutant QM 9978. The results of this study provide evidence that induction of gene expression by cellulose does not necessarily correlate with that by sophorose.