The Nile tilapia represents one of the most widely cultured fish species in Egypt. In recent years, the Egyptian tilapia farms have been suffering from unknown high mortalities during the summer season resulting in high economic losses. Investigation of mortalities by different research groups revealed a significant association with various microbial agents, however, the main causes of summer mortality in tilapia are still unknown. The current study aimed to identify the possible causes of summer mortalities in tilapia farms from three main tilapia-producing governorates in Egypt. Clinical, microbial, histopathological and environmental examinations have been carried out over a period of a year in different tilapia farms and results were recorded. Clinical examination of moribund and recently dead fish showed extensive cutaneous haemorrhages with scales losses and erosion, abdominal distension, and ulcers. Screening of fish samples by viral and bacterial examination and molecular assays, including PCR and sequencing, showed negative results for tilapia lake virus and positive detection of Aeromonas hydrophila subsp. dhakensis, A. sobria, A. veronii, A. enteropelogenes, and Streptococcus iniae.  The current study may contribute to our knowledge of potential causes of summer mortality in tilapia and would help in the development of control strategies for the devastating tilapia losses in the Egyptian fish farms.