Abstract  The potential of the biocontrol agent Trichoderma harzianum strain T-203 to induce a growth response in cucumber plants was studied in soil and under axenic hydroponic growth conditions. When soil was amended with T. harzianum propagules, a 30% increase in seedling emergence was observed up to 8 days after sowing. On day 28, these plants exhibited a 95 and 75% increase in root area and cumulative root length, respectively, and a significant increase in dry weight (80%), shoot length (45%) and leaf area (80%). Similarly, an increase of 90 and 30% in P and Fe concentration respectively, was observed in T. harzianum inoculated plants. To better characterize the effect of T. harzianum during the early stages of root colonization, experiments were carried out in a gnotobiotic hydroponic system. An increased growth response was apparent as early as 5 days post-inoculation with T. harzianum, resulting in an increase of 25 and 40% in the dry weight of roots and shoots, respectively. Similarly a significant increase in the concentration of Cu, P, Fe, Zn, Mn and Na was observed in inoculated roots. In the shoots of these plants, the concentration of Zn, P and Mn increased by 25, 30 and 70%, respectively. Using the axenic hydroponic system, we showed that the improvement of plant nutritional level may be directly related to a general beneficial growth effect of the root system following T. harzianum inoculation. This phenomenon was evident from 5 days post-inoculation throughout the rest of the growth period, resulting in biomass accumulation in both roots and shoots.