Nutritionally balanced mixture of ingredients provided to fish, support the maintenance, growth, reproductive performance, flesh quality and health of the animals at an acceptable cost. The feed supplementation affects the water quality and culture systems. Aflatoxins are a family of ex- tremely toxic, mutagenic, and carcinogenic compounds produced by Aspergillus flavus and A. parasiticus. The present study was planned to study the effect of aflatoxin B1 on growth and his- tology of various development stages of Catla catla. For this purpose three trials were conducted on the different age groups i.e. advanced fry, fingerlings and on fingerling fish intra peritonially. Aspergillus parasiticus. National Road Racing League (NRRL) 2999 was grown on rice for afla- toxin production in Quality Operation Lab. Institute of Biochemistry and Biotechnology (Uni- versity of Veterinary and Animal Sciences Lahore). A total number of 25 healthy Catla catla weighing 150±10 gm were collected from Fish Farm of the University and maintained in glass aquaria. The fish were supplied with a commercial fish ration (fish meal, maize gluten meal, soyabean meal, sunflower meal, rice polish, molases) previously analyzed without detectable af- o latoxin B1. The water temperature was kept at 29±2 C throughout the experiment. Aflatoxin B1 was administered intra peritoneally beneath the pelvic fin in each Catla catla weighing 150 gm. Dosages prepared were 0.09 ml aflatoxin B1/1cc distilled water, 0.180ml/1.5 cc distilled water, 0.271ml/2 cc distilled water, 0.361ml/2.5 cc distilled water 0.451 ml/3 cc distilled water which was here after regarded as 10 ppb, 20ppb, 30ppb, 40ppb, 50ppb respectively and served as dif- ferent treatments. Control group, however, did not receive any aflatoxin B1. Blood was sampled over 24 hours, 48 hours, 72 hours, 96 hours and 120 hours and collected from the caudal vein, gills and heart using a medical syringe previously rinsed with 2.7% ethylene diamine tetra acetic acid (EDTA) solution for laboratory analysis. Haematological parameters, like red blood cells (RBCs), white blood cells (WBCs), erythrocyte sedimentation rate (ESR), haemoglobin (HB), albumen (ALB), and globulin(GLB) levels, A:G and Total Protein were determined. The result showed that Catla catla is very susceptible to aflatoxin intoxication. If injected intraperitoneally its toxicity spread immediately to each and every part of its body, hold control of each organ, destabilize, and disrupts its function changing its products or making of its own will. The cas- cade of events totally damages the functioning and viability of an organ ultimately succumbing the animal. In the 2nd experiment, advanced fry Catla catla were used as an experimental animals and studies were conducted for the period of three months. There were 5 treatments and a control with two replicates in each.180 advanced fry were randomly stocked having 15 No. each glass aquaria. 30% crude protein containing feed ingredients such as fish meal, maize gluten meal, soya bean meal, sunflower meal, rice polish and molasses were analyzed for the presence or absence of tox- ins by qualitative and quantitative methods. Control group was free of aflatoxins while the re- maining 5 treatment received aflatoxin in the order of 10, 20, 30, 40 and 50 ppb hereafter termed as T1, T2, T3, T4 and T5. All the fishes in various groups were regularly and equally fed at the rate of 5% of their body weight daily. Response to feed was measured in growth increments and his- topathological damages to the tissues /cells in vital organs. All the water quality parameters were monitored on fortnightly basis. At the end of the experiment, all fishes were harvested weighed, measured and dissected for histopathological studies. The result showed that the advanced fry of Catla catla were more susceptible as compared to fingerlings. Fishes in control group grew much better than those exposed to aflatoxin containing feed. In the 3rd trial, fingerling of Catla catla were stocked in the aquariums for the period of three months. In this trial aflatoxin produced in the trial was mixed in feed at various proportions to elucidate its effect on growth performance of fingerlings Catla catla. For this experiment 10 fin- gerling were stocked into twelve glass aquaria containing twenty liter of water. One control without aflatoxin B1 while other five test groups with duplicate received aflatoxin B1. After twelve week experimental period, liver, kidney and intestine were collected from the control and aflatoxin B1 exposed groups for histopathological studies of fingerlings. Feed ingredients such as fish meal, maize gluten meal, soya bean meal, sunflower meal, rice polish and molasses were analysed for the presence or absence of toxins by qualitative and quantitative methods. The re- sults showed that Catla catla is very susceptible to aflatoxin intoxication. If administered orally its toxicity spread slowly and steadily with the increments of dosage. The control aquarium showed the best growth while growth decreased from T1 to T5 when aflatoxin B1 dose increased in the feed. This cascade of events totally damaged the functioning and viability of an organ ul- timately succumbing the animal to death.