Effects of chronic exposure to alpha and gamma iron oxide nanoparticles (alpha-Fe2O3 and gamma-Fe2O3 NPs) were investigated through exposure of tilapia (Oreochromis niloticus) to 0.1, 0.5 and 1.0 mg/L (9.2 x 10(-4), 4.6 x 10(-3) and 9.2 x 10(-3) mM) aqueous suspensions for 60 days. Fish were then transferred to NP-free freshwater and allowed to eliminate ingested NPs for 30 days. The organs, including gills, liver, kidney, intestine, brain, spleen, and muscle tissue of the fish were analyzed to determine the accumulation, physiological distribution and elimination of the Fe2O3 NPs. Largest accumulation occurred in spleen followed by intestine, kidney, liver, gills, brain and muscle tissue. Fish exposed to gamma-Fe2O3 NPs possessed significantly higher Fe in all organs. Accumulation in spleen was fast and independent of NP concentration reaching to maximum levels by the end of the first sampling period (30th day). Dissolved Fe levels in water were very negligible ranging at 4-6 mu g/L for alpha-Fe2O3 and 17-21 mu g/L for gamma-Fe2O3 NPs (for 1 mg/L suspensions). Despite that, Fe levels in gills and brain reflect more dissolved Fe accumulation from metastable gamma-Fe2O3 polymorph. Ingested NPs cleared from the organs completely within 30-day elimination period, except the liver and spleen. Liver contained about 31% of alpha- and 46% of gamma-Fe2O3, while spleen retained about 62% of alpha- and 35% of the gamma-polymorph. No significant disturbances were observed in hematological parameters, including hemoglobin, hematocrit, red blood cell and white blood cell counts (p > 0.05). Serum glucose (GLU) levels decreased in treatments exposed to 1.0 mg/L of gamma-Fe2O3 NPs at day 30 (p < 0.05). In contrast, GLU levels increased during the elimination period for 1.0 mg/L alpha-Fe2O3 NPs treatments (p < 0.05). Transient increases occurred in glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), and lactate dehydrogenase (LDH). Serum Fe levels did not change during exposure (p > 0.05), but increased significantly within elimination period due to mobilization of ingested NPs from liver and spleen to blood. Though respiratory burst activity was not affected (p > 0.05), lysozyme activity (LA) was suppressed suggesting an immunosuppressive effects from both Fe2O3 NPs (p < 0.05). In contrast, myeloperoxidase (MPO) levels increased significantly in treatments exposed to alpha-Fe2O3 NPs (p < 0.05), and the effect from gamma-polymorph was marginal (p > 0.05). The results indicate that morphological differences of Fe2O3 NPs could induce differential uptake, assimilation and immunotoxic effects on O. niloticus under chronic exposure. (C) 2016 Elsevier B.V. All rights reserved.