Two experiments were carried out in calcareous sandy loam soil of Experimental Farm of the Faculty of Environmental Agricultural Science, El-ArishSuez CanalUniversity, North Sinai Governorate, Egypt. The first experiment (pot experiment) aimed to study the effect of deficit irrigation water on enumeration of microorganisms associated with roots of wheat (Triticum aestivum vulgare cv. Sakha 93) and bacterial diversity. The obtained data indicated that there is no obvious effect of deficit irrigation water on the enumeration of total bacterial and actinomycetes count. Bacterial strains were classified into 14 clusters according to morphological characteristics, gram staining and growth curves. The bacterial strains were grouped into ten clusters when soils were irrigated by 100 and 75% ETm whereas fourteen clusters were obtained when soils were irrigated by 50 and 25% ETm causing an appearance of new strains having the ability to survive in water stress conditions. The second experiment (field experiment) aimed to investigate the effect of deficit irrigation water under drip irrigation system on water relations, yield and growth parameters of wheat crop. The actual evapotranspiration (ETa) of wheat crop was decreased with increasing applied deficit irrigation water. The highest ETa value of wheat crop was found at applied deficit irrigation water of 100% ETm followed by 75, 50 and 25% ETm. The highest water use efficiency was found at deficit irrigation water level of 25% ETm followed by100, 50, 75% ETm. The deficit of irrigation water of 100 % ETm had the highest values for plant height and number of tillers per m², while the deficit irrigation water of 25 % ETm had the lowest ones. The deficit of irrigation water of 100 % ETm had the highest values of grains, straw yields and weight of 1000 grain, while the deficit irrigation water of 25 % ETP had the lowest ones.