The life history of the sea lamprey is briefly outlined. Mature adults are anadromous, spawning chiefly in May and June and dying shortly thereafter. The larvae pass 4 to 5 years in the sand, gravel, and silt banks of the spawning stream before transforming into the parasitic stage. After becoming fitted for a parasitic life, they live as free-swimming adults, parasitic on fish, from 1 1/2 to 3 1/2 years before becoming sexually mature and ready to spawn and die.