The aim of the present study was to determine median lethal concentrations (LC 50) of unionised ammonia (NH3) for the larvae and fingerlings of Tilapia (Oreochromis niloticus L. 1758). The mean 48 h LC50 values were determined as 1.009 +/- 0.02 mg/l for larvae and 7.40 +/- 0.01 mg/l for the fingerlings. Larvae exposed to different concentrations of ammonia moved very rapidly, lost equilibrium in water and showed sideways swimming. In the case of the fingerlings, an increase in their movements, ventilation, convulsions, spiral swimming, efforts to swallow air from the surface of water, increase in the mucous secretion in the gills and on the body surface, haemorrhage in the gills, and darkening in the eye and on the skin were observed. The histological study showed hyperplasia and fusion of the lamella on the gills.