The sea lamprey (Petromyzon marinus) is widely distributed in the Great Lakes but it is absent from or scarce in large parts of the watershed. Since 1957, larval sea lampreys have been detected in only 433 (7.5%) of the 5747 streams in the Great Lakes basin. Parasitic-phase sea lampreys range throughout the lakes, wherever suitable host fishes occur, but probably do not inhabit the western and central basins of Lake Erie to any great extent during summer. Many environmental conditions influence the distribution of sea lampreys. Streamflow and water temperature are of major importance in attracting spawning runs to streams. The dispersal of spawning adults within streams is influenced mainly by blockages, water temperature, current, bottom type, and the presence of inland lakes. Water temperature is probably the most important factor affecting the development and survival of embryos. The distribution of larval lampreys is limited primarily by barriers that block adult spawning runs, warm temperatures, low and unstable flows, hard stream bottom, and pollution; nonetheless, larvae have been found in a wide range of habitats exhibiting these conditions. Interconnecting waterways and attachment to fishes and boats are considered major factors in the lake movements of parasitic-phase lampreys.Key words: sea lamprey, Petromyzon marinus; Great Lakes, geographic distribution, influences, movement, spawning, larvae, parasitic, control