This study was carried out on 150 Oreochromis niloticus at Dakhalia governorate within winter and summer seasons of the year 2015. The clinicohistopathological picture and biochemical investigations of the diseased fish were recorded. Sixty fish were infected with various types of bacterial species, of which, (49) fish were infected with Gram-negative bacteria while (11) fish were infected with Gram-positive one. The most common bacterial infection were Aeromonas hydrophila (33%), Pseudomonas fluorescens (20.5%), Streptococcus agalactiae (17.8 %), Aeromonas sobria (12.3 %), Vibrio anguillarum (9.5%) andVibrio alginolyticus (6.8%). The highest total prevalence of bacterial infections was recorded in summer season (39%) compared to 21 % in winter. Isolates varied in their antibiotic sensitivity pattern. Inflammatory changes associated with these bacterial diseases were obvious in histopathological sections. Disease conditions were affected by various environmental stressors especially temperature, pH, salinity, ammonia and some heavy metals.