The present work was performed to study the effect of unionized ammonia (UIA) on mortality rate, clinical signs, postmortem findings, some biochemical and immunological parameters and histopathological findings of Oreochromis niloticus challenged with Clostridium perfringens type A. The experiment was carried out on 160 Oreochromis niloticus with average body weight 45±5 gram which were divided into 8 equal groups. Groups (1-4) were inoculated intraperitoneally with 0.2 ml/ fish of sterile cooked meat broth, while, groups (5-8) were inoculated intraperitoneally with 0.2 ml/fish of 24 hours old culture of Clostridium perfringens type A (0.5x10⁷ CFU) on cooked meat broth. The groups (1 and 5) were used as a control (not subjected to UIA) but the groups (2 and 6), (3 and 7) and (4 and 8) were subjected to 0.53, 0.265 and 0.132 mg/L of UIA. The results revealed that the highest mortality rate was 80% and 70% in infected fish exposed to 0.53 and 0.265 mg/L of UIA, respectively. The fish showed nervous manifestation, effort to swallow air from the water surface and mortality occurred with open mouth with dark body coloration, hemorrhages on body and increased mucus secretion on skin and gills. The levels of serum alanine aminotransferase (ALT), creatinine and cortisol were significantly increased, while, the level of serum immunoglubulin M (IgM) was significantly decreased. Also, some histopathological changes were recorded in liver, kidney, gills and intestine. In conclusion, the elevated levels of UIA increased the virulence of Clostridium perfringens type A infection in Oreochromis niloticus