The study was carried out on 1000 Tilapia (O. niloticus) collected from private and governmental semi intensive earth pond (Central Laboratory for Aquatic Research laboratory (Abbassa) in Sharkia Province, through the seasons of the year 2008. The clinical signs and post mortem lesions of diseased fish were recorded. Results of the study revealed that (721) and (80) fish were found to be infected with different types of parasites and fungal infection respectively. The isolation and identification of parasites in infected fish including the following, 1-Protozoa: (Trichodina sp. in a percentage (42%), of total parasitic disease in fish examined. The research showed that Trichodina sp. represented the highest rate of infection was observed in summer season (64.8%).         2-Trematodes: a-Monogenia (Cichlidogyrus liberianus and Cichlidogyrus arthracanthus (28%), the highest rate of infection was observed in spring season (38%). b-Digenean encysted metacercaria (Centrocestes (5.9%), Clinostomum (5.2%), Euclinostomum (4.9%)),and the highest rate of infection were observed in spring (6.4%), winter (7.2%) and spring (8.8%) respectively 3-Nematodes: a-Acanthocephala (7.5%) and the highest rate of infection was observed in autumn season (8.8%). b- Contracaecum (3%) and the highest rate of infection was observed in autumn season (4.8%). 4-Crustacea (Ergasilus (4.9%) and the highest rate of infection was observed in spring season (8.8%). The isolation and identification of mycotic diseases in infected fish revealed that the Seasonal prevalence of saprolegniosis showed that the highest rate of infection was observed in winter season (43.7%). From this study it could be concluded the wide spread of parasitic diseases and to lesser extent the mycotic one in cultured O. niloticus under semi intensive system in Sharkia province which may be due to the improper management practice in such farms. The results revealed that late spring and summer are critical times for the occurrence of parasitic diseases, At the same time the rigorousness of saprolegnisis on O. niloticus   specially in winter season which is associated with severe mortalities and high economic losses must be considered. So the application of the ideal system of management including good quality water, proper nutrition and using of immunstimulant will decrease the negative impacts of such diseases in their critical times of occurrence.