Once released in the environment, engineered nanoparticles (NPs) can undergo important transformation resulting in changed properties under natural conditions. This study investigated the fate, the bioavailability and the immunotoxicity of cerium oxide (CeO₂) nanoparticles in fish exposed to CeO₂ in representative surface waters differing in pH, organic matter content and conductivity (green and brown waters). Following an incubation period of NP CeO₂ in different surface waters, particle size distribution and shape were determined by ultrafiltration and ICP-mass spectrometry, electronic microscopy and dynamic light scattering (DSL). Bioaccumulation and effect biomarkers focusing on the immune system responses (viability of immune cells and phagocytic activity) were also determined. Particle size distributions significantly changed under all types of surface waters where aggregation of NPs was commonly observed. Indeed, >90% of NPs CeO₂ were found as aggregates (>450 nm) and large colloids (>100 nm). Less than 1% cerium (Ce) was found in the truly dissolved fraction (<1 kDa) suggesting no evidence of degradation for NP CeO₂ in the water samples after 96 h. The NPs CeO₂ were preferably accumulated in fish gills and accumulation was the highest in green waters which contained less total organic carbon (TOC), higher conductivity (218 μS/cm) and higher pH (7.8-8.0) than brown waters. The toxic properties (induced phagocytosis) of NP CeO₂ also differed when  dispersed in brown, green and tap waters. NPs CeO₂ induced fish mortality at initial concentration of 10 μg/L Ce in both tap and green waters but not in brown waters which have different and high organic matter sources, lower pH and conductivity values. In conclusion, NPs CeO₂ tends aggregate in representative freshwater, adsorb on gills and the immunotoxic potential is reduced in the presence of high natural organic matter, mildly acidic pH and low conductivity as found in brown water;