Imidacloprid is a neonicotinoid insecticide with neurotoxic action that, as a possible alternative for commonly used organophosphorus pesticides, has gained registration in about 120 countries for use in over 140 agricultural crops. Only few data are available on its toxicity for soil invertebrates. We therefore assessed the effects of imidacloprid on survival, weight gain, feeding rate, total protein content, glutathione S-transferase activity (GST), and digestive gland epithelial thickness in juveniles and adults of the terrestrial isopod Porcellio scaber. After two weeks of feeding on imidacloprid-dosed food, weight gain (NOEC 5μg/g dry food) and feeding rate (NOEC 10μg/g) in juveniles, and feeding rate (NOEC<10μg/g) and digestive gland epithelial thickness (NOEC<10μg/g) in adults were most affected. In juveniles induction of GST activity and increase of total protein content per wet animal weight was detected at 5μg/g dry food, whereas in adults a reduction of GST was observed at 25μg/g (NOEC 10μg/g). An estimate of actual intake rates suggests that imidacloprid affects isopods at similar exposure concentrations as insects. The toxicity of imidacloprid was similar to that of the organophosphorus pesticide diazinon, tested earlier using the same methods Stanek, K., Drobne, D., Trebse, P., 2006. Linkage of biomarkers along levels of biological complexity in juvenile and adult diazinon fed terrestrial isopod (Porcellio scaber, Isopoda, Crustacea). Chemosphere 64, 1745–1752. At actual environmental concentrations, diazinon poses a higher risk to P. scaber. Due to its increasing use in crop protection and higher persistence in soil, imidacloprid might however, be potentially more dangerous after long-term application. We conclude that toxicity testing with P. scaber provides relevant, repeatable, reproducible and comparable toxicity data that is useful for the risk assessment of pesticides in the terrestrial environment.