Fishes have socio- economics position in all over the world particularly marine fish give a bulk of food for mankind even as fishes of the lakes and rivers are also important source for the food supply. Fish provide a high protein, low-fat diet, which is low in saturated fats. Now a day, due to different disease such as diabetes, cardiovascular problems, hypertension, osteoporosis, osteomalacia, ocular diseases etc., and doctors recommend white meat as compared to red meat. Fish is also recommended for people suffering from malnutrition. Fish production is an important source of income and employment and plays a major role in Pakistan’s economy. Its organ such as liver is an important source of oil and has medicinal value and is used for the treatment of different diseases. Fish also provide fish fertilizer, fishmanure and several other products of commerce. Some fish take pollutants from the environment and water in which they live in and the food they eat .Some of these contaminants provoked in the form of tumourous growth. The word tumour has often been used in a general way to signify any hump or swelling. But commonly the word tumour define as a new growth of cells which deviated from normal surroundings cells in shape, size, structure and organization. The present study deals with neoplastic growth and as well as trematode infection. The intensity of infection was noted for the whole period of investigation from January 2009 to December 2012. During the study 2,373 species of fish were examined, out of which 1,178 were found to be infected by helminths. Mostly the trematodes and neoplastic growth were observed in fish Johnius dussumieri (Valenciennes, 1833) called as “Mushka” of Karachi coast. In case of trematodes the maximum prevalence was recorded during the hot (summer) season of the period while the minimum prevalence was observed in cold season. There were such a great variations observed during the period of study especially in case of neoplastic growth which varies in all the months. Statistical analysis was carried out by Descriptive Statistics, Descriptive Measures and Analysis of variance and Duncnan’s Multiple Range Test which revealed that there were significant difference in the monthly results of mean intensity of trematodes and as well as neoplastic growth. Gross pathological study shows that tumour was separated from the surrounding organs and showed no sign of proliferation. No haemorrhages were observed on the tumour or around the organs. Histopathologically the tumour shows hyperplasia, malpighian cells, atrophied cells, high prismatic basal cells, and fibrosis with eosinophilia and inflammatory cells infiltration. It is concluded that presence of trematode infection not only is responsible for various types of histopathological changes on the fish individually but it may also responsible to provoke bio-chemical changes to promote tumourous growth in most of the fishes under study. But some of the fishes under study shows only tumourous growth without any trematode infections which may be due to some pollutants which contaminate the water or the surrounding environment which are responsible for such type of growth in fishes.