The use of only one or a few species-representing an entire taxon-in ecotoxicological standard tests poses risk of underestimating the impact of toxicants on the environment. In earthworm ecotoxicity tests, the species Eisenia fetida or Eisenia andrei are commonly used, and there is evidence that these species respond relatively insensitive towards environmental pollution. With the present study, we wanted to evaluate the risk of underestimating effects of the insecticide imidacloprid in soil organisms by comparing E. fetida with two other earthworm species (Aporrectodea caliginosa and Lumbricus terrestris) regarding their sensitivities towards soil contaminated with this widely used insecticide. In laboratory experiments, the specimens were individually exposed to various concentrations of the pesticide (0.2, 0.66, 2 and 4 mg kg(-1) dry weight (DW)) for 1, 7 and 14 days. Afterwards, histopathological changes in the midgut, chloragogenous tissue and skin, as well as body mass changes, were assessed. While significant changes in body mass in E. fetida and A. caliginosa occurred after exposure to imidacloprid concentrations as low as 0.2 (7 days) and 0.66 mg kg(-1) DW (14 days), significant body mass changes in L. terrestris observed to 2 and 4 mg kg(-1) DW, for 7 and 14 days of exposure, respectively. The histopathological examinations revealed that significant cellular changes already occurred after 24 h exposure to the lowest test concentrations in all species, but the degree of detrimental effects as well as species-specific differences were dependent on the monitor tissue. In general, E. fetida seemed to be more sensitive than L. terrestris concerning cellular alterations, but the hierarchy in species-specific differences was less obvious than for body mass change. Even if E. fetida proved to be the most sensitive species in this study, general differences in sensitivity make evident that always a range of species-being representatives of an animal taxon-in ecotoxicological tests should be tested in order to avoid underestimations of effects. In the case of testing only one species, an increase of safety factors should be considered. Since effects already occurred at environmentally relevant concentrations, the use of imidacloprid in agriculture might be of great concern.