Brook silversideLabidesthes sicculusand inland silversideMenidia beryllinacoexisted in Lake Conroe, Texas, for at least 6 years before we began our field sampling in 1980. We hypothesized that an extensive infestation of aquatic vegetation was a critical factor allowing sympatry, because introduced inland silversides have rapidly displaced endemic brook silversides in other, less-vegetated, reservoir systems. We believe that an abundant invertebrate prey base associated with the aquatic vegetation was responsible for a low level of interspecific competition for food resources between the two silverside species in Lake Conroe. After removal of vegetation in 1981–1983 by grass carpCtenopharyngodon idella, the inland silverside population expanded and the brook silverside population declined. By 1986, brook silversides were rare in seine samples taken from, or near, the main basin of the reservoir and appeared to be restricted to tributaries and coves. Data suggest that food habits of the silverside species diverged after vegetation removal, coincident with declines in the limnetic and littoral invertebrate prey bases. The observed shifts in diets, abundances, and distributions of the two silverside populations were consistent with results predicted by studies on silverside feeding mechanics and the competitive exclusion principle.