Renewed activity in the placer gold mining country of the Pacific Coast States has raised the question of how much harm silt does to salmon and trout. The effect silt may have on migrating adult fish, on the selection of spawning places, and on the survival of fish food, is discussed. The conclusion is drawn that silt, whether from placer mining or natural erosion, is harmful to salmon and trout if it is heavy enough to form a layer on the stream bottom or if it persists during periods between floods.