A number of 360 apparently healthy and naturally infected Nile tilapia with body weight range (40-80) grams were obtained from Abassa fish farms, Sharkia Governorate, Egypt, random collected from pond with uncovered cement ponds (A) and greenhouse cement ponds (B) (180 from each) during winter season of 2013 /2014 ( December and January). Water parameters were recorded at the collection time namely temperature, ammonia and dissolved oxygen. At the present work tilapia fish were subjected to clinical and post mortem examination. All Fish were examined for isolation and identification of bacterial, mycotic and parasitic pathogens and histopathological examinations were performed on infected fish tissue.
The isolated bacteria were Pseudomonas aeruginosa, Pseudomonas fluorescens, Proteus vulgaris, Escherichia coli, Enterobacter intermedius, Enterobacter aerogenes, Enterobacter cloacae and Aeromonas hydrophila. The bacterial prevalence was higher in pond B (57.8%) than in pond A (33.3%).
Mycological examination revealed that fungal isolation of four genera were isolated from pond (A) (Penicillium spp, Aspergillus niger , Blastomyces spp  and Scopulariopsis brevicaulis ) but in pond (B) were positive to fiver genera (Penicillium spp, Aspergillus niger , Blastomyces spp  and Scopulariopsis brevicaulis ). The fungal isolates were higher in pond (B) 77.8% than in pond (A) 55.6%.
The examination of tilapia fish revealed the presence of four different parasitic agents which were identified, as Myxobolus tilapiae, Trichodina heterodentata and two encysted metacercariae (EMC) Centrocestus formosanus in gills and Heterophyes sp in muscles .The parasitic agents was higher in pond (B) 77.8% than in pond 55.6%(A).
The results of the histopathological examination of naturally infected fish were also discussed.
It was concluded that it be good to using greenhouse in fish culture to avoid the decrease in water temperature during winter season but other factors must be taken in consideration like water sources to avoid spread of diseases.