The aim of this work is to study the effect of host biological factors on the different parasitic helminthes in Oreochromis niloticus and Tilapia zilli from Lake Manzala. The sex, weight and length of fish were determined. The examination of fish revealed the presence of seven distinct defined metacercariae in addition to undifferentiated metacercariae and nematode larvae. The recovered metacercariae were Centrocestus sp., Haplorchis sp., heterophyes sp., Stictodora sp., Pygidiopsis sp., Echinostoma sp. and Phagicola sp. Results showed that O. niloticus was more susceptible to parasitic infection than T. zilli. The high est prevalence of infection was found in larger fish (weight and length) and in males (Sex) in the case of O. niloticus, while the highest prevalence of infection was found in smaller fish (weight and length) and in females (sex) in T. zilli. The occurrence of each of the helminthes gives a different response to these factors as discussed in this work.