The generally accepted 2% ratio of transmitter weight to body weight constrains or precludes telemetry studies examining the timing and location of spawning of small adult westslope cutthroat trout Oncorhynchus clarkii lewisi in headwater streams. We empirically determined effects of surgically implanted dummy transmitters ranging in weight from 1 to 5 g on the swimming stamina and growth of small (81.3–206.9 g) adult westslope cutthroat trout in the laboratory to establish acceptable transmitter weights for field studies on this species. Mean growth rates and swimming stamina were not significantly different among treatments, including controls. No precipitous decline or threshold beyond which performance deteriorated markedly was observed. Data collected using telemetered westslope cutthroat trout implanted with transmitters less than about 4% of body weight should therefore approximate information about untelemetered individuals without significant bias. However, we also detected subtle effects on growth within this transmitter weight range related to individual transmitter–body weight ratios (0.5–5.3% initially), and there were indications that swimming stamina was affected similarly. Each 1% increase in transmitter– body weight ratio elicited an 11.6% decrease in growth and a possible 5.6% decrease in swimming stamina at 6 weeks postimplantation. Therefore, transmitter selection should weigh the costs of increased transmitter weight on fish performance against the benefits of longer transmission durations. In the case in which transmitter weights approaching 4% of body weight are necessary to complete a study and slight decreases in performance are not expected to affect findings materially, such weights may be acceptable. In other cases, researchers should choose the lightest possible transmitters that allow study goals to be achieved and not automatically select transmitters weighing 4% of body weight.