Competition among coho salmon fry in stream aquaria supplying natural drift was found to reflect the history of exposure of eggs and resulting fry to dissolved oxygen concentration prior to emergence. Size disparities induced by differential hypoxial stress were amplified with time, and fry that had been exposed to the most severe hypoxial conditions were most prone to emigrate. Most emigrants placed in an initially vacant, replicate system remained there, grew rapidly, and became as large as, or larger than, nonemigrants. Size of former emigrants reflected enhanced feeding opportunity due to less competition for food and space in the replicate system. Competition was referred more precisely to hypoxial history by using net production. A replicate population fed hatchery food provided a comparison. When exposed to the stream aquaria, this population substantiated the previous findings. Diel cycles of activity and aggression peaked at dawn and dusk, and were related to competition for food and space. The ecological significance of the results is discussed with particular regard to competition for food and space, and the effect of a size-related social order that put smaller individuals at a disadvantage.