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Previous work has shown that spherical CuO nanomaterials show negative effects on cell and animal physiology. The biological effects of Cu 2 O materials, which possess unique chemical features compared to CuO nanomaterials and can be synthesized in a similarly large variety of shapes and sizes, are comparatively less studied. Here, we synthesized truncated octahedral Cu 2 O particles and characterized their structure, stability, and physiological effects in the nematode worm animal model, Caenorhabditis elegans . Cu 2 O particles were found to be generally stable in aqueous media, although the particles did show signs of oxidation and leaching of Cu 2+ within hours in worm growth media. The particles were found to be especially sensitive to inorganic phosphate (PO 4 3− ) found in standard NGM nematode growth medium. Cu 2 O particles were observed being taken up into the nematode pharynx and detected in the lumen of the gut. Toxicity experiments revealed that treatment with Cu 2 O particles caused a significant reduction in animal size and lifespan. These toxic effects resembled treatment with Cu 2+ , but measurements of Cu leaching, worm size, and long-term behavior experiments show the particles are more toxic than expected from Cu ion leaching alone. These results suggest worm ingestion of intact Cu 2 O particles enhances their toxicity and behavior effects while particle exposure to environmental phosphate precipitates leached Cu 2+ into biounavailable phosphate salts. Interestingly, the worms showed an acute avoidance of bacterial food with Cu 2 O particles, suggesting that animals can detect chemical features of the particles and/or their breakdown products and actively avoid areas with them. These results will help to understand how specific, chemically-defined particles proposed for use in polluted soil and wastewater remediation affect animal toxicity and behaviors in their natural environment.