AbstractThe effects of acidic medium on the fry of Cyprinus carpio have been studied. The eggs of Cyprinus carpio were obtained from the Bilawali Fish Farm, Indore M.P. in July 1976. The eggs were put in various concentrations of HCl ranging from 0.0025 ppm to 0.0500 ppm. One specimen from each experimental group was sacrificed for histological studies every week. The microtomy sections were studied under high power. It was found that when the fries were kept in acidic medium following changes took place: Nucleus became inconspicuous and cell wall became disorganised; muscle layers of gut wall became indistinct and disorganised to some extent; epithelial lining of the gut was partially destroyed. vacuolation in the connective tissue of gut was seen due to destruction of some of the cells; hepatic cells of the liver were destroyed at various spots to a varying degree. Comparative study of results showed that the percentage of hatching is inversely proportional to the strength of pollutant (HCl) (Tab. 1).Table 1 shows that the LD‐50 period of fries is inversely related to the strength of HCl in ppm and that the maximum number of fries overturned on the 6th or 7th day of the experiments.Statistical analysis of results indicates that the correlation coefficient between strength of pollutant and LD‐50 period is −0.877. It means that by increasing the strength of the HCl the survival period of fries decreased.Even at 0.0025 ppm (lowest concentration) HCl is toxic to the fries of Cyprinus carpio.