A combination of 100 mg/ml of ketamine hydrochloride (Ket) and 20 mg/ml of xylazine hydrochloride (Xyl) was used to immobilize coypus (Myocastor coypus). Eight mature coypus (four males and four females) were injected intramuscularly with doses ranging from 2.33 to 6.25 mg/kg of KET and 0.25 to 0.86 mg/kg of Xyl. The mean (±SE) time for induction, arousal, and recovery were 7.3 ± 2 min, 23.5 ± 0.3 min and 46 ± 2.5 min, respectively. The mean ± SE doses injected were 4.07 ± 0.52 mg/kg Ket (range, 2.33 to 6.25 mg/kg) and 0.5 ± 0.08 mg/kg Xyl (range, 0.25 to 0.86 mg/kg). No adverse responses were observed in any of the animals treated.