A quantitative survey of the submersed aquatic macrophytes of Devils Lake, a 151 ha softwater seepage lake, was conducted July 30–August 1, 1984, in an effort to assess historical changes in the lake's macrophytes and water quality. Biomass (oven dry weight) and frequency of occurrence data were obtained from 28 transects spaced 200 m apart. Sample quadrats of 0.1 m2were collected at 5 m intervals along transects from shore to a water depth of 9 m. Eurasian watermilfoil,Myriophyllum spicatum, ranked second only toPotamogeton robbinsiiin importance, comprising 21 percent of total biomass and 35 percent frequency of occurrence. Milfoil was visually dominant, forming three distinctive surface canopy beds 25–50 m wide by 300 m long at water depths from 2–3 m. Total milfoil acreage (3 ha) represented 10 percent of the total vegetational cover in the lake. Average milfoil biomass within dense stands ranged from 160 to 183 g/m2between beds. Milfoil expansion within Devils Lake was accompanied by a decline inElodea canadensis. M. spicatummay be a disturbance indicator species (pioneer) rather than an aggressive invader.