ATER stress is one of the most important abiotic stresses that …….....may limit agriculture production worldwide. This work was carried out on mango trees (Mangifera indica L.) to study the effect of exposure to different levels of drought stress (65, 75, 85 and 100 % of full irrigation requirements), in addition, to evaluating the role of using some plant growth promoting rhizobacteria (PGRP); such as Azospirillum and Azotobacter, in alleviating drought-induced changes. Physiological and biochemical changes were determined in mango leaves after two seasons of different treatments. Results indicated that membrane stability, photosynthetic pigments and insoluble sugar contents were significantly decreased with increasing drought levels, while electrolyte leakage, soluble sugars, total carbohydrates and proline content were sharply increased compared to control. Lipid peroxidation level and the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) enzymes showed progressive increases with drought levels. Application of biofertilizers may be effective in alleviating the adverse effect of water stress. Bio-fertilizers caused marked increase in photosy nthetic pigments and carbohydrate contents and a decrease in proline content compared to control. Keywords: Drought, Water stress, PGPR, Mangifera indica,Chlorophyll, Antioxidant enzymes, Proline.Abbreviations: PGRP, plant growth promoting rhizobacteria; ROS, reactive oxygen species; SOD, superoxide dismutase; POD, peroxidase; CAT, catalase; EL, Electrolyte leakage; M SI, membrane stability index; DW, dry weight; FM , fresh mass; M DA, M alondialdehyde; S, Azospirillum; B, Azotobacter.Countries in arid and semi-arid regions suffer from water shortage for agriculture usage. Water stress affects plant growth and productivity as it causes various physiological and biochemical changes including hormonal and nutritional imbalance, ion toxicity, desiccation, abscission, senescence and susceptibility to diseases (Nadeem et al., 2014). Also, drought can lead to pigment degradation (Hendry et al., 1987) causing irreversible damage to the photosynthetic apparatus (Clarke et al., 1996). R.R. KHALIL et al. Egypt. J. Bot., Vol. 56, No. 2 (2016) 472On the other hand, water stress can cause rapid damage to plant cells membrane due to an uncontrolled enhancement of reactive oxygen species (ROS) (Moussa and Abdel-Aziz, 2008). Excess accumulation of ROS may initiate destructive oxidative processes such as lipid peroxidation and ch lorophyll bleaching as well as oxidation of proteins, deoxyribonucleic acid and carbohydrates (Ashraf, 2009). The degree of damage caused by ROS depends on the balance between the production of ROS and its removal by efficient antioxidant scavenging system which includes nonenzymic and enzymic antioxidants (Azooz et al., 2009). The enzymic antioxidants include superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase and enzymes of ascorbateglutahione (AsA-GSH) cycle such as ascorbate peroxidase, monodehydroascorbate redu...