The St. Marys River is believed to be the primary source of sea lampreys (Petromyzon marinus) in Lake Huron. Planning or evaluating lampricide treatments required knowing where lampricides could effectively be placed and where larvae were located. Accurate maps of larval density were therefore critical to formulating or evaluating management strategies using lampricides. Larval abundance was systematically assessed with a deepwater electrofishing device at 12,000 georeferenced locations during 1993 to 1996. Maps were produced from catches at those locations, providing georeferenced detail previously unavailable. Catches were processed with a geographic information system (GIS), to create a map of larval density. Whole-river treatment scenarios using TFM (3-trifluoromethyl-4-nitrophenol) were evaluated by combining the map with one of lethal conditions predicted by a lampricide-transport model. The map was also used to evaluate spot treatment scenarios with a granular, bottom-release formulation of another lampricide, Bayluscide (2',5-dichloro-4'-nitro-salicylanilide). Potential high-density plots for Bayluscide treatment were selected from the map and estimates of area, cost, and larval population were developed using the GIS. Plots were ranked by the cost per larva killed. Spot treatments were found to be more cost effective than a conventional TFM treatment and Bayluscide was applied to 82 ha in 1998 and 759 ha in 1999. Effectiveness was estimated with stratified-random sampling before and after treatment in 1999 at 35%. Ten percent already had been removed in 1998, for a total reduction of 45% percent. This marked a change in how research and planning were combined in sea lamprey management to minimize treatment costs and evaluate success.