This study demonstrates the acute toxicity of lanthanum oxide nanoparticles (La sub(2)O sub(3) NP) on two sentinel aquatic species, fresh-water microalgae Chlorellasp. and the crustacean Daphnia magna. The morphology, size and charge of the nanoparticles were systematically studied. The algal growth inhibition assay confirmed absence of toxic effects of La sub(2)O sub(3) NP on Chlorellasp., even at higher concentration (1000 mg L super(-1)) after 72 h exposure. Similarly, no significant toxic effects were observed on D. magnaat concentrations of 250 mg L super(-1) or less, and considerable toxic effects were noted in higher concentrations (effective concentration EC sub(50) 500 mg L super(-1); lethal dose LD sub(50) 1000 mg L super(-1)). In addition, attachment of La sub(2)O sub(3) NP on aquatic species was demonstrated using microscopy analysis. This study proved to be beneficial in understanding acute toxicity in order to provide environmental protection as part of risk assessment strategies.