Ronstar was applied at concentrations of 2.0,5.75 and 9.5 mg/L. Histological alterations were determined in liver and kidney of the catfish Clarias lazera.The histological changes in liver of catfish C lazera intoxicated with different doses of Ronstar, included dilation of the blood veins and sinusoids, vacuolation in the cytoplasm and pyknosis in the nuclei of the hepatocytes, haemorrhage is also noticed in the veins and sinusoids.The phagocytic Kupffer and endothelial cells are also abundant as an immunoresponse reaction. The vacuolation is the most frequent histological alteration observed at all tested doses of Ronstar.Ascorbic acid (lOmg/lkg of body *weight)was reported to reduce some of the histological damage occurred in the liver tissue. It alleviated the rate ofpyknosisand vacuolationThe rate of ascorbic acid healing effect depends on the Ronstar tested dose.Ronstar induced different histological alterations in the kidney of the catfish C lazera The glomeruli became expanded and their capillaries were extended, the subcapsular space enlarged or disappeared. The haemopoietic tissue exhibited some vaculation and the renal tubules were swollen or degenerated. Some of them took an irregular shape .The expanded glomeruli with extended capillaries  were the most frequent histological alteration observed in the different doses of Ronstar .Ascorbic acid induced some amelioration in the histological structure of kidney at all tested doses of Ronstar. It decreased the rate of glomerular expansion or caused a restoration to its normal shape, rate of the extending of the glomerular capillaries was also decreased.The subcapsular space restored its normal appearance. The rate of vacuolation was alleviated in the haemopoietic tissue, while some of the renal tubules tend to be normal.At higher doses of the Ronstar, the healing effect of ascorbic acid became less potent.