Excluding motor boats from small experimental plots in a lake with heavy boat traffic significantly increased macrophyte biomass, coverage, and shoot height compared to impacted areas. Lakes across the country are increasingly threatened by overuse, particularly involving more and larger motor boats. In recent years, Lake Ripley, located in Jefferson County in southern Wisconsin, has exhibited a decline in macrophyte abundance along with increased motor boat activity. To examine the possible connections between these two events, a study on the effects of motor boats on submerged aquatic macrophytes was conducted during the summer of 1995. Four enclosures, two of solid plastic and two of mesh fencing, were built in each of two areas in about 1 m of water adjacent to high boat traffic areas. These enclosures were intended to exclude motor boat access and, in the solid-walled enclosures, to block the turbidity generated by boat-induced sediment resuspension. Weekly water samples were collected for turbidity, suspended solids, dissolved oxygen, and temperature. At the end of the study, plant biomass, height and percent cover were measured inside the enclosures and in control plots. Our hypothesis was that motor boats limit macrophyte growth either directly by physical disturbance or more indirectly by generating turbidity which decreases light availability and therefore limits production and biomass. Results indicated that motor boats reduced plant biomass quite dramatically, primarily through scouring of the sediment substrate and direct cutting, but not through turbidity generation.