Fe3O4 nanoparticles' accumulation in aquatic organisms, especially fish, and their induc-tion of various pathological changes in the host organism have raised significant con-cerns. In our study on common carp, subjected to two different doses of Fe3O4 nanopar-ticles (10 and 100 mg per 10 g of food) for a 7-day period, notable alterations were observed in the intestine and liver. At a lower dose (10 mg), distinct changes like villi disintegration in the small intestine and cytoplasmic structure pathology in enterocytes were evident. Liver examination revealed alterations in erythrocytes, hepatocytes, cana-liculi, and bile ducts. With a higher dose (100 mg), destructive changes occurred in all layers of the small intestine and the liver. Electron microscopy confirmed the sequential entry and bioaccumulation of Fe3O4 nanoparticles, starting from microvilli and travers-ing various cellular organelles. The observed nanoparticle size in fish intestine and liver structural elements was consistently up to 20 nm. These findings underscore the poten-tial risks associated with Fe3O4 nanoparticles, emphasizing the need for further investi-gation into their environmental impact and implications for aquatic ecosystems.