Carfentanil citrate and thiafentanil oxalate have been used successfully to immobilize captive and free-ranging ungulates. The objective of this study was to compare the efficacy and certain physiologic parameters of protocols by using the 2 opioids in gaur (Bos gaurus). Eight adult gaur bulls were immobilized for electroejaculation at Omaha's Henry Doorly Zoo (Omaha, Nebraska, USA). All the animals were immobilized twice, by using each of the following protocols one time: 10 mg carfentanil combined with 100 mg xylazine (CX), reversed with 1,000 mg naltrexone and 24 mg yohimbine; and 12 mg thiafentanil combined with 20 mg medetomidine (TM), reversed with 120 mg naltrexone and 100 mg atipamezole. Immobilization drugs were delivered intramuscularly into the shoulder area via pole syringe. Electroejaculation was carried out by a standardized protocol to duplicate procedural stimulation on each animal. Induction and recovery times, initial rectal temperature, heart rate, respiratory rate, anesthetic depth, oxygen saturation, indirect blood pressure, and arterial blood gases were recorded at the time of initial handling, before ejaculation, and after ejaculation. Antagonists were administered 1/4 i.v. and 3/4 s.q. Both protocols require a small volume of drug for a large ungulate, provide smooth induction, and adequate anesthesia. Both protocols produced a significant hypoxemia, although the animals on CX showed slightly better blood gas values (based on lower partial pressure of carbon dioxide) and numerically lower blood pressure values. Animals on TM had better muscle relaxation and smoother recoveries, with no renarcotization noted. The results of the present study indicate the TM and CX protocols used for immobilizing gaur result in similar quality ejaculates that can be used for fertility examination as well as for assisted reproduction such as artificial insemination. Additional immobilizations need to take place to further compare these 2 combinations in this species.