A standardized bioassay using the yellow dung fly, Scathophaga stercoraria L. (Diptera: Scathophagidae), was developed to test the lethal and sublethal toxicity of parasiticide residues in livestock dung. The repeatability of the bioassay was assessed for the parasiticide, ivermectin, in thirteen tests performed in seven laboratories in Germany, the UK, Switzerland and Canada. Test results had an acceptable range of heterogeneity. The calculated concentration at which 50% egg-adult mortality was observed (Effect Concentration (EC50) averaged 20.8 +/- 19.1 (Standard Deviation (SD)) microg ivermectin / kg fresh dung (range: 6.33 - 67.5). Mortality was not observed below an average calculated No Observable Effect Concentration (NOEC) of 8.1 +/- 7.7 microg / kg. However, prolonged developmental time, and in a subset of tests reduced body size, was observed above an average calculated NOEC of 0.8 +/- 0.8 microg / kg. An oviposition choice test revealed further that yellow dung fly females do not discriminate among dung of different ivermectin concentrations. We conclude that the yellow dung fly is suitably sensitive, and the methods sufficiently repeatable, to support use of this standardized bioassay by the international community in the registration of new veterinary pharmaceuticals.