Ocean-going ships carry, as ballast, seawater that is taken on in port and released at subsequent ports of call. Plankton samples from Japanese ballast water released in Oregon contained 367 taxa. Most taxa with a planktonic phase in their life cycle were found in ballast water, as were all major marine habitat and trophic groups. Transport of entire coastal planktonic assemblages across oceanic barriers to similar habitats renders bays, estuaries, and inland waters among the most threatened ecosystems in the world. Presence of taxonomically difficult or inconspicuous taxa in these samples suggests that ballast water invasions are already pervasive.