The promoters of constitutive genes have proven themselves to be useful parts of expression vectors for genetic engineering in different organisms. Based on a differential screening of an induced cDNA library, we have used a new approach to isolate constitutively expressed genes which can be used for this purpose. Following this strategy, we have isolated constitutively expressed cDNA clones of the mycoparasitic fungus Trichoderma harzianum. One of these cDNA clones corresponds to a gene (cob4) that encodes a novel serine + alanine-rich protein. The predicted amino acid (aa) sequence of this protein (171 aa, deduced M(r) 15,564, pI 12.49) displays a motif that is also found in structural proteins of different species and in a yeast thermoinducible protein. Northern (RNA) blot analysis has demonstrated that cob4 is expressed during growth when glucose or cell walls of a phytopathogenic fungus are provided as carbon source. Hybridizations of T. harzianum genomic DNA indicate that this gene most probably belongs to a multigene family.