The present study was carried out to determine the possible causes of summer mass mortalities among farmed Nile tilapia, Oreochromis niloticus. Pure bacterial colonies were isolated from moribund O. nitoticus from 13 different fish farms; which suffered from high mortalities ranged from (50-80%), during the period from April to October, 2018. Fish showed external hemorrhagic spots, skin darkening, abdominal distension and exophthalmia. Internally, congestion and enlargement of internal organs with serous or hemorrhagic fluid was the most obvious picture. Based on the phenotypic and biochemical characterization using API20E, the isolated bacteria were identified as (A. veronii, A. hydrophila, A. caviae and A. sobria; Aeromonads), (Ps. Fluorescence; Pseudomonas spp), (E. sakazakii and E. cloacae; Enterobacter spp), (C. freundii; Citrobacter spp), (S. odorifera, S. liquefaciens, and S. marcescens; Serratia spp), (S. lutiensis, S. equine; Streptococcus spp), Lactococcus lactis and Proteus vulgaris; with the most prevalence to Aeromonads. Most isolates were accurate identified by PCR and gene sequencing. Water physicochemical parameters were measured at the farm site; which showed an increase in the level of both pH and ammonia. In order to confirm the pathogenicity of the bacterial isolates, an experimental infection was conducted using different doses. The results revealed that A.veronii (HY2) at dose of 9×108 cells/ml was the most pathogenic , with mortality rate 100 %. This study concludes that A. veronii, C. freundii, P.vulgaris and P. fluorescens are implicated in summer mortality of Nile tilapia, without neglecting the role of water quality in worsening the problem.