The insensitive munitions compound 3-nitro-1,2,4-triazol-5-one (NTO) was recently approved by the U.S. Army to replace cyclotrimethylene trinitramine (RDX) in conventional explosives. As its use becomes widespread, concern about the potential toxicity of NTO increases. NTO can undergo microbial reduction to 3-amino-1,2,4-triazol-5-one (ATO), which is recalcitrant in waterlogged soils. In this study, the acute toxicity of NTO and ATO towards various organisms, including microorganisms (i.e., methanogenic archaea, aerobic heterotrophs, and Aliivibrio fischeri (Microtox assay)), the microcrustacean Daphnia magna (ATO only), and zebrafish embryos (Danio rerio), was assessed. NTO was notably more inhibitory to methanogens than ATO (IC50 = 1.2 mM, > 62.8 mM, respectively). NTO and ATO did not cause noteworthy inhibition on aerobic heterotrophs even at the highest concentrations tested (32.0 mM). High concentrations of both NTO and ATO were required to inhibit A. fischeri (IC20 =19.2, 22.4 mM, respectively). D. magna was sensitive to ATO (IC50 = 0.27 mM). Exposure of zebrafish embryos to NTO or ATO (750 mu M) did not cause lethal or developmental effects (22 endpoints tested). However, both compounds led to swimming behavior abnormalities at low concentrations (7.5 mu M). The results indicate that the reductive biotransformation of NTO could enhance or lower its toxicity according to the target organism. (C) 2017 Elsevier B.V. All rights reserved.