The beneficial and deleterious effects of five isolates of Trichoderma harzianum and seven isolates of T. longibrachiaturn, on the stand and some agronomic traits of flax, were evaluated under field conditions in 2013/2014 and 2014/2015 growing seasons. In each season, seeds of flax cultivar Giza 10 were treated with a dry powder of Trichoderma -sorghum mixture at a rate of 10 g/kg. seeds. Field evaluation revealed poor and inconsistent performance among the tested isolates from one season to another. Thus, while certain isolates effectively controlled seedling blight and improved some agronomic traits in one season, they were ineffective or even increased the disease in the other season. Due to the unstable and poor performance of Trichoderma isolates, they were considered either ineffective or at the most part effective in controlling flax seedling blight and improving agronomic traits. In each season, grouping the isolates by cluster analysis, based on their effect pattern, was not related to their morphological taxonomy.