Due to the un-sufficient information about susceptibility of Caspian roach (Rutilus caspicus) to environmental ammonia, in the present study mortality rate, clinical and histological signs of this fish in acute exposure situation to un-:union:ized ammonia have been studied. To this 180 fingerling with average weight of 6.5 ± 1 were divided in 18 aquaria (10 fish per aquarium) and acclimatized for 2 weeks. After adaptation 5 treatment groups and one control group with three replicates for each one were considered. Fish of treatment groups were exposed to different concentration including 2.5, 5, 10, 15 and 20 mg/l of total ammonia which were equal to 0.34, 0.66, 1.32, 1.98 and 2.66 mg/l of un- :union:ized ammonia. No ammonia exposure was happened in control group. According to the results 96 h median lethal concentration (LC₅₀) of :union:ized ammonia in fingerlings Caspian roach calculated as 1.62 mg/l. Congestion in the kidneys and petechial haemorrhage in basal part of eyes were the typical clinical signs. Histological studies were characterized by congestion in kidney, oedema and epithelial lifting in secondary lamella in gill tissues and diffused degeneration in kidney tubules. No mortality and abnormal clinical and histological change were observed in fish of control groups. According to the present study maximum sub lethal concentration of :union:ized ammonia has calculated as 0.162 mg/l for Caspian roach fingerling.