Through examination of 176 samples of the fresh water fish Liza abu obtained from Tigers River in Mosul city 2005, the percentage of total internal parasites infection was 57.38%. They were the trematode larval stage Diplostumum spathaecum 44.88%; the cestode larval stage of Ligula intestinalis 32.95%, oocyst of Cryptosporidium spp. 28.97%, oocyst of Eimeria spp. 23.29%. Cryptosporidium was recorded for the first time in Mosul city during examination of intestinal contents. They appeared after staining with modified Zeihl- Neelsen as bright red spherical oocysts with a diameter of 3-7 µ. The effect of season on types of infection was also studied.