The Mississippi Silverside, Menidia audens, was first encountered in Kentucky Reservoir, the most downstream reservoir on the mainstem Tennessee River, in 1991. Over the next 19 years, it invaded all nine mainstem reservoirs, consisting of 1049 river km, and two tributary reservoirs, at an average rate of 55 km per year. This species was first encountered in Barkley Reservoir, the lowermost reservoir of the Cumberland River, in 1990, and has since established populations throughout at least 402 river km of the mainstem Cumberland River. Following the first record of M. audens in an unimpounded section of a Tennessee River tributary during 1998, 1048 sites were sampled throughout the Tennessee River system and this species was documented in only eight additional unimpounded tributaries, indicating aversion to lotic systems. Following the invasion of Tennessee River reservoirs by M. audens, mean abundance of the Brook Silverside, Labidesthes sicculus, linearly decreased with time. Electrofishing catch per unit effort (CPUE) data indicate that 14 years after sympatry with M. audens, L. sicculus were undetectable or occurred at extremely low densities in reservoirs. Reservoir populations of L. sicculus exhibited cyclic variation in abundance among years prior to the presence of M. audens. The observed range of electrofishing CPUE of L. sicculus before invasion by M. audens (n  =  3865) was significantly different (P < 0.001) than the observed range following the presence of M. audens (n  =  3975). After detection of M. audens, electrofishing CPUE of L. sicculus decreased and the range of variation from year to year diminished. Data from Cumberland River reservoirs showed similar patterns of displacement.