The intraspecific competitiveness of a genetically engineered strain of Trichoderma virens was assessed relative to the non-transformed, progenitor strain and an isogenic, auxotrophic strain using a replacement series design. The transformed strain was less fit, but appeared more competitive than the wild type or the auxotroph in this assay. To validate this finding and to evaluate the ability of a strain to establish in an environment occupied by a competitor another experimental approach was developed. In various treatments the transformed strain was unaffected or only slightly inhibited by a competing strain. In contrast, the wild type and the auxotrophic strain were strongly inhibited by the presence of the transformed strain. These findings support the conclusion that this genetically engineered strain is more competitive than the wild-type strain and thus may be persistent in the environment.