A glyceraldehyde-3-phosphate dehydrogenase (gpd) cDNA was isolated from the filamentous fungus Trichoderma harzianum in the course of a search for light-regulated genes in this organism. There is apparently only one copy of gpd in the T. harzianum genome, and its sequence is most similar to that of other filamentous ascomycetes. Trichoderma grows in the soil as a saprophyte or mycoparasite. A brief pulse of blue light, or nutrient depletion, induces sporulation, which is accompanied by altered patterns of abundance of specific polypeptides. Mycoparasitic development is also accompanied by changes in gene expression. The abundance of gpd mRNA decreased strongly during sporulation, and was lowest in samples consisting o mature conidiophores and conidia. When T. harzianum was grown in the presence of cell walls of the phytopathogen Rhizoctonia solani, the gpd mRNA level was much lower than in similar cultures grown on glucose. The repression of gpd, which is usually considered a constitutively expressed gene, may be part of the switch to sporulation or to the simulated mycoparasitic state. The implications of these findings for the use of gpd promoters to confer high constitutive expression are discussed.