Juvenile fish belonging to three genera and ten species of salmonids, were compared in field and laboratory studies. The main objectives were to describe the aggressive social behavior in a comparative way and to study those components of behavior associated with stream residence. In comparisons of aggregating tendencies of lake trout, rainbow trout, cutthroat trout, coho salmon and chum salmon it was found that chum fry formed relatively tight schools while the others did not. Coho fry aggregated when feeding and dispersed afterwards. Both rainbow and cutthroat showed a preference for shadowed portions of their containers. Rainbow selected the deep end of a tank graded from shallow to deep, while chum fry swam back and forth. Rainbow showed no selection for a small waterfall. It was shown that food, predatory fish and dominant individuals affected the selection of a location. From these experiments it was reasoned that resident and migrant fish have different innate environmental preferences but that these can be modified by external factors. The rate of locomotion was shown to be highest in migrant chum fry, lowest in resident fishes and intermediate in lake trout. The continuous swimming of chum fry was contrasted with the discontinuous swimming of residents. Again lake trout were intermediate. Standard observations were made of intraspecific groups in the laboratory. Resident fishes were aggressive and developed social hierarchies based on fighting, displays and nipping. Most nipping was performed by dominant fish and all of the activities of the subordinates were influenced by the presence of the dominant individual in their group. Some migrant fishes were aggressive, others were not. Chum fry nipped and chased each other but, evidently because of their continuous swimming movements, did not develop stabilized dominance orders. Lake trout, sockeye fry and pink fry were not aggressive and did not develop social hierarchies. Dominant coho fry attacked intruders more vigorously than they did members of their own group and the intruders often died. It was observed that intruders showed a characteristic "escape behavior" which may have identified them and singled them out for attack. In field studies carried out during different seasons, it was observed that individual coho fry exchanged positions and moved about within a home range. They did not appear to restrict themselves to private territories. During winter floods the home range was vacated and the fish occupied pools along the stream edge. In spring pre-migrant smolts were in pools while newly emerged fry were in shallow margins. Predation by smolts appeared to affect the location of the fry. Interspecific combinations were observed in the laboratory. Rainbow trout were most aggressive. The aggressive activities of spawning adults were similar to those of the juveniles but the actual reproductive movements were unique to the adults. Agonistic behavior appeared to be primitive in lake trout with increasing specialization through dolly varden to brook trout. It was highly developed with specific variations in displays in brook trout, cutthroat trout, rainbow trout and coho salmon. The other species within the genus Oncorhynchus exhibited a degeneration in residential behavior. / Science, Faculty of / Zoology, Department of / Graduate