Green sturgeon Acipenser medirostris spend much of their lives outside of their natal rivers, but the details of their migrations and habitat use are poorly known, which limits our understanding of how this species might be affected by human activities and habitat degradation.We tagged 355 green sturgeon with acoustic transmitters on their spawning grounds and in known nonspawning aggregation sites and examined their movement among these sites and other potentially important locations using automated data-logging hydrophones. We found that green sturgeon inhabit a number of estuarine and coastal sites over the summer, including the Columbia River estuary, Willapa Bay, Grays Harbor, and the estuaries of certain smaller rivers in Oregon, especially the Umpqua River estuary. Green sturgeon from different natal rivers exhibited different patterns of habitat use; most notably, San Francisco Bay was used only by Sacramento River fish, while the Umpqua River estuary was used mostly by fish from the Klamath and Rogue rivers. Earlier work, based on analysis of microsatellite markers, suggested that the Columbia River mixed stock was mainly composed of fish from the Sacramento River, but our results indicate that fish from the Rogue and Klamath River populations frequently use the Columbia River as well. We also found evidence for the existence of migratory contingentswithin spawning populations.Our findings have significant implications for the management of the threatened Sacramento River population of green sturgeon, which migrates to inland waters outside of California where anthropogenic impacts, including fisheries bycatch and water pollution, may be a concern. Our results also illustrate the utility of acoustic tracking to elucidate the migratory behavior of animals that are otherwise difficult to observe. ?? American Fisheries Society 2011.