The current investigation was undertaken to determine the potential causes of summer mass mortalities among farmed Nile tilapia (Oreochromis niloticus). Pure bacterial colonies were isolated from moribund O. niloticus from 13 different fish farms 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. The phenotypic and biochemical characterization using API20E identified the bacterial isolates 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 accurately identified by PCR and gene sequencing. Water physicochemical parameters were measured at the farm sites showed an increase in the pH and ammonia levels. 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 9×10 8 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 Nile tilapia summer mortalities, without neglecting the role of water quality in worsening the problem.