The present work aimed to study the effect of stocking density and feeding rates of Oreochromis niloticus on water quality and abundance of phytoplankton in earthen ponds. This experiment was conducted in 12 earthen ponds at Parsik Culture, Idko, Behira Governorate, Egypt. The ponds have the same area of about 2 feddan with an approximately average depth of 1m. Monosex fries of Oreochromis niloticus reared for 214 days with different stocking densities and feeding rates (T₁: stocking density of 6 fish/m³ with feeding rate of 2.5%; T₂: stocking density of 6 fish/m³ with feeding rate of 3.5%; T₃: stocking density of 8 fish/ m³ with feeding rate of 2.5% and T₄: stocking density of 8 fish/ m³ with feeding rate of 3.5%), each treatment had three replicates. Results showed that the highest average values of salinity(‰), total dissolved solids (mg/l), hydrogen ion concentration, total hardness (mg/l) and total nitrite (mg/l) were recorded in T₁ and T₂ (low stocking fish densities); while their lowest values were recorded in T₃ and T₄ (high stocking fish density). On the other hand, the highest average values of chlorophyll a (μg/l) were recorded in T₃ and T₄ and their lowest values were recorded in T₁ and T₂.The highest average value of dissolved oxygen content (mg/l) was recorded in T₂ and T₄ (high feeding rate), while the lowest average was recorded in T₁ and T₃ (low feeding rate). Total nitrate (mg/l) and orthophosphate had the reverse trend of these results. The maximum average value in abundance of phytoplankton specimens was recorded in T₄ (222728667±37584559 organism/l) and the lowest occurred in T₂ (156813467±22022381 organism/l). The variations in phytoplankton abundance were generally non-significant between the different experimental treatments. Four divisions of phytoplankton were recorded in the experimental fish ponds; Chlorophyceae, Cyanophyceae, Bacillorophyceae and Euglinophyceae. The most abundant division in all experimental fish ponds was Chlorophyceae (42.46%); followed by Euglinophyceae (33.85%).The variations in abundance of these phytoplankton divisions were statistically significant.The phytoplankton abundance was greatly varied in each treatment ponds during different months of the experiment. Generally, it reached the highest averages during the period of September-November in all treatments, and the lowest averages were recorded during June. The results concluded that the highest stocking density (8fish/m³) with low feeding rate (2.5%) was positively correlated with best water quality and high abundance of phytoplankton in earthen ponds of Oreochromis niloticus.