The poppy stem gall wasp, Iraella luteipes (Thompson) (Hymenoptera: Cynipidae), is one of the main pests of the opium poppy, Papaver somniferum L., an economically important pharmaceutical crop cultivated worldwide. In a previous study, we obtained from I. luteipes larvae a strain of the entomopathogenic fungus Beauveria bassiana (Ascomycota: Hypocreales) that can become established endophytically in opium poppy plants. A field experiment was conducted to study the ability of this B. bassiana strain to provide systemic protection against damage by I. luteipes in opium poppy in southern Spain for three seasons. Conidial suspensions were applied as seed dressings, leaf sprays, or soil sprays. The effect of the treatment was studied by harvesting fully ripened plants and dissecting I. luteipes larvae from the stem. The effect of treatment on growth and yield was also evaluated. Emergence of I. luteipes adults was not uniform over the 3 yr, with important differences exhibited in the duration of the emergence period, although the flight peaks tended to occur in mid-late April. B. bassiana seed dressings, leaf sprays at the fourth true-leaf stage, and soil sprays were not significantly different in their ability to reduce the number of larvae per plant compared with the controls, with percentage reductions of 36.5–58.5, 64.4–73.4, and 51.9–57.2% in 2005, 2006, and 2007, respectively. Even though the population level of I. luteipes increased over the 3 yr, the efficacy of the fungal inoculation in reducing the larval population was maintained throughout the study period. No significant differences between inoculation methods were detected in the percentage of leaf pieces showing fungal growth when placed on B. bassiana selective medium, with mean values in the range of 10–15% for the three seasons. Leaf pieces from controls did not exhibit any sign of B. bassiana growth when placed on B. bassiana-selective medium. Neither adverse effects on growth and yield nor symptomatic tissues were observed in B. bassiana-treated plants compared with controls in any of the three seasons.