An investigation was conducted for food and feeding habits, nematode parasites and histopathology of the infected intestine of fish to get information on this subject as relatively little work was carried out previously. A total of 1500 fishes belonging to 10 genera, 14 species, 10 families and 6 orders were collected from Fish harbor, Karachi coast, Pakistan. The microscopic examination and Steroscan electron microscopy (SEM) of cephalic and tail regions of nematodes was carried out. The nematode specimens belonging to 3 genera, including Procamallanus (Spirocamallanus), Dujardinascaris and Spirocotyle were identified and species are described here. Tissue damage associated with nematode infection was also studied in two fish species. Various stages of necrosis, degeneration and morphological alteration in villi of the intestinal tissues, involving whole thickness of the bowl wall was observed. This information is useful for parasitologists and fish pathologist, students and teachers and research workers in this field of study as well as fish farming agencies.