We observed the behaviour of juvenile steelhead trout (Oncorhynchus mykiss) in two rivers in British Columbia, Canada, to determine the importance of invertebrate drift abundance, intruder pressure, and body size of territory holders as correlates of territory size. Territory size increased with increasing fish size, but fish smaller than 5 cm had relatively large territories for their body size, in comparison to fish that were larger than 5 cm in length. After statistically controlling for the effect of body size, territory size was inversely related to the abundance of drifting invertebrates. Territory size was not related to the number of intrusions observed on a territory per hour, but was inversely related to local fish density, a second measure of intruder pressure. Steelhead trout appear to defend territory areas that are most similar in size to juvenile Atlantic salmon. This study provides evidence that stream-dwelling salmonids scale territory size to body size in a manner that allows them to capture a maximum daily ration of food on their territories.