We used both on-shore holding and field releases of wild fish to evaluate the effect of using intracoelomic surgical implantation of small radio transmitters (4 g, 9.5 mm diameter, 26 mm length) on adult mountain whitefish. In the holding study, all fish survived the 48 hr period following surgery. Significant mortalities were noted on days four and five of the experiment; however, tagged fish were no more likely to die than control fish. Incisions showed signs of healing and had no macroscopic inflammation. The times until exhaustion during forced swimming trials were similar between tagged and control fish. However, after fish were held for five days, exploratory activity levels were depressed in both groups, emphasizing the potential consequences of retaining this species for even short periods of time. When we implanted twelve mountain whitefish with tags (0.7–3.0% of body weight) and released them in the Columbia River, we had reasonable success at tracking movements of these fish over three seasons and there was no evidence of tagging mortality after release. We suggest that using the protocols described here, it is possible to implant adult mountain whitefish with electronic tags but advise that fish should not be held for more than several hours post-implantation to minimize stresses of handling and captivity. This finding is consistent with the growing body of literature suggesting that pre- and post-surgical care is an important component of successful transmitter implantation.