Haematological and histological changes in carp, Cyprinus carpio after exposure to low environmental pH for 96 hr. have been investigated. Water acidity clearly influenced all blood values of carp. Red blood cell counts, haemoglobin content and haematocrit percentages were higher at low environmental pH than those of controls. Plasma total protein, glucose and cholesterol increased also after acid-exposure. Plasma sodium and calcium concentrations decreased with increasing pH while plasma potassium was increased. Histological studies showed that gills had much higher numbers of chloride cells. Liver showed extensive vacuolation of parenchyma probably due to abnormal accumulation of lipid. Kidney showed extensive tubular cell disintegration and pyknosis nuclei are present side by side with swelling one. It has been suggested that, oxygen deprivation and severe ionic imbalance are the cause of fish death in acidic water.