Previous reports have indicated the absence of rainbow trout as native in Montana. All planted rainbow trout in Montana came indirectly from the McCloud River in California and were originally derived from a coastal steelhead trout population. Electrophoretic techniques can distinguish between inland and coastal steelhead trout. Using these techniques, along with evidence from hatchery records and geographical distribution, it is shown that rainbow trout are native in Callahan Creek and in the Yaak River. This is important in the consideration of fish planting measures because the gene pool of these natural populations could be jeopardized by plantings of hatchery stocks, thus eliminating fish which should be better adapted to the Montana enviornment.