Copper, lead and zinc were measured in the organs (muscles, gills and gonads) of Oreochromis niloticus inside and outside fish cages in River Nile at Damietta Region from five different ecological sites throughout the period from spring 1996 to spring 1997. The obtained data were subjected to analysis of variance (one-way, two-way and three-way anova) in order to find the significant differences of metal concentration in fish organs wjth respect to inside and outside cages, seasons and sites. Present results revealed that metal concentration in fish organs inside and outside fish cages, respectively exhibited the following descending order: Cu (μgg wet weight), gonads (5.27±0.87 and 4.45±0.86) > gills (4.17±0.47 and 2.37±0.27) > muscles (l.49±0.18 and O.99±0.16); Pb (μg/g wet weight),gills (3.94±0.39 and I .54±0.39) > gonads (2.77±0.60 and I .29±0.25) >muscles (I .92±0.23 and O.87±0. 12) and Zn (μg/g wet weight), muscles (14.8±0.2 and 13.2±1.2) > gills (12.3±2.4 and 7.89±0.55) > gonads (10.2±0.7and 6.82± 0.2I). Concentration of tested heavy metals in fish organs followed a sequence of: Zn > Cu> Pb. Concentration of heavy metals
in O. niloucus organs inside cages was significantly higher than that outside cages. The seasonal and site variation of tested metals concentration in organs of O. niloticus inside and outside cages revecaled strong significant differences ( p < 0.05). Except copper in muscles, the tested metals concentration in fish organs showed a similar seasonal trend
with their concentration in water and sediment. Site I and III and sometime IV were the most polluted sites by heavy metals. Copper and zinc levels in O . niloticus organs (except gonads) lie within the safe limits. On the other
hand, lead concentrations exceeded the permissible maximum world level in fish organs.