الفهرس 
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Abstract
The parasitic disease of fish constitute the most common problem in fish farms in Egypt.
The present work deals with the study of endoparasites which infected three species of the most common popular Egyptian freshwater fish, Clarias lazera, Oreochromis aureus and Oreochromis niloticus collected from concrete and earthen fishponds of Central laboratory for Aquaculture Reasearch at Abbassa, Sharkia Governorate in the period extended from the beginning of Jan. 2002 till the end of Dec. 2002.
Out of 435 examined fish specimens, 159 (36.5%) were infected with helminth parasites.
The infection rate was highest in Clarias lazera (58%) followed by Oreochromis aureus (35.5%) and Oreochromis niloticus (16%).
The collected parasites included six species of trematodes (Orientocreadium batrachoides, Acanthostomum species, Eumasenia aegyptiacus, Gauhatiana batrachii, Paramasenia aegyptiacus, Lepocreadium species), one species of encysted metacercaria (Euclinostomum heterostomum), three species of cestodes (Monobothrioides species, Polyonchobothrium clarias and Bothriocephalus species), two nematodes (Procamallanuis laeviconchus and Paracamallanus cyathopharynx) and one Acanthocephala (Acanthosentis tilapae).
In the present work Clarias lazera was infected with trematodes with an infection rate (20%) while it was (3.33%) and (2.96%) in Oreochromis niloticus and Oreochromis aureus respectively.
Cestodes were recorded only in Clarias lazera at a precentage (18.7%).
Also, nematodes were found only in Clarias lazera with an infection rate (14.66%).
Acanthocephala was recorded in Oreochromis niloticus and Oreochromis aureus at a precentage (35.55% and 16%) respectively.
Concerning the seasonal prevalence, it was found that, the highest peak of infection was in summer (52.9%) autumn (39.83%), spring (30.7%) and lowest one was observed in winter (26%).
The peak of trematodes infection rate was in summer (17.2%) and the lowest infection rate was in winter (0%), while in cestodes the highest insestation rate was in winter (12%), the lowest infection rate was in spring (3.1%).
In nematodes the highest infection rate was obtained in summer (12.6%) and the lowest one was in autumn (2.5%).
Regarding, acanthcephala the highest infection rate was in autumn (21.2%) while the lowest one was in winter (11%).
Moreover, the taxonomy and morphological status of each collected parasite was include in the present work, with notes on the histopathological changes.