Allelic variation at 10 microsatellite loci was assayed in steelhead trout (Oncorhynchus mykiss) scale samples (n=333) collected in various years 1976-2003, from the lower mainstem Kitimat River, British Columbia. The objective was to investigate the genetic structure of natural steelhead populations in the Kitimat River before hatchery stocking started in 1984 (baseline samples 1976-1977, 1983-1984, n=145), and to assess potential genetic impact of in-river interbreeding of returning cultured adult fish with wild spawners over almost 20 years of large- scale hatchery operation (1987-2003, n=188).