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A knowledge of fine-scale population genetic structure and patterns of dispersal is an essential component of any action to conserve genetic diversity and maintain population viability. We genotyped 417 juvenile steelhead Oncorhynchus mykiss from the main stem and tributaries of the Bulkley-Morice River, British Columbia, at 10 microsatellite loci to assess fine-scale population structure and the patterns and magnitude of juvenile dispersal and mixing. We detected significant genetic structuring among juvenile steelhead from seven tributaries of the Bulkley-Morice River (pairwise F-ST: 0.008-0.156) and found significant isolation by distance among the tributary populations (R-2 = 0.198, P = 0.038). These results reflect the homing behavior of spawning adults as well as the temporal stability of those populations. Genotype assignment of tributary-caught juveniles showed that rates of juvenile dispersal varied among tributaries. The assignment of juveniles sampled from the main stem of the river to source tributary populations suggested that long-distance movement in juvenile steelhead is common and that juveniles are well mixed in the main stem. Dispersal and fine-scale genetic structure in pristine steelhead populations are more complex than previously thought. Therefore, actions to conserve Bulkley-Morice River steelhead must strive to maintain the genetic diversity of tributary populations.