ABSTRACT Nine transformants of Trichoderma longibrachiatum with extra copies of the egl1 gene were studied for mitotic stability, endoglucanase production, and biocontrol activity against Pythium ultimum on cucumber seedlings. The transformants showed a significantly higher level of expression of the egl1 gene in comparison to the wild type under both inducing and noninducing growth conditions. Transformants with the egl1 gene under the control of a constitutive promoter had the highest enzymatic activity. Both the endoglucanase activity and the transforming sequences were stable under nonselective conditions. When applied to cucumber seeds sown in P. ultimum-infested soil, T. longibrachiatum transformants with increased inducible or constitutive egl1 expression generally were more suppressive than the wild-type strain.