Veterinary antibiotics may find their way into the aquatic environment through direct or indirect pathways due to their widespread use. Fenbendazole is a benzimidazole anthelmintic that is widely used in veterinary medicine. To evaluate the potential ecological risk of fenbendazole, we examined the molecular and biochemical responses of biomarker genes such as heat shock proteins (HSPs), cytochrome P450 (CYP450), glutathione S-transferases (GSTs) and hemoglobins (Hbs) in Chironomus riparius for long periods. The expression of HSP70, HSP40, HSP90 and CYP450 in C. riparius increased significantly after exposure to all concentrations of fenbendazole evaluated, while the levels of GST and HbA only increased in C. riparius exposed to relatively high concentrations of fenbendazole (30 microg L(-1)). HbB expression did not differ significantly between the control and treatment groups. Exposure to 30 microg L(-1) fenbendazole had significant effects on the survival, growth, sex balance of emergent adults and development of mouthpart deformity in C. riparius. These results should constitute an important contribution to the understanding of the toxicology of fenbendazole in C. riparius. Moreover, the responses of the biomarker genes also provide valuable information that will aid in understanding the effects of fenbendazole in aquatic ecosystems.