The range of brook silverside (Labidesthes sicculus) extends from Minnesota eastward to southern Quebec, and south through New York, Texas, and Florida. The species is found in the St. Lawrence and Richelieu Rivers in Quebec, and the Allegheny and St. Lawrence River drainages, Mohawk and Hudson Rivers in New York. We found brook silverside for the first time in four of the five basins of Lake Champlain, over a range of 150 km, in 1998. Several individuals were collected at each site, indicating that the species is established in the lake. The route of introduction of brook silverside is unclear; they could have entered the lake from the Mohawk River via the Champlain Canal at the south end or from the Richelieu River via the Chambly Canal at the north end. This finding extends the range of brook silverside by approximately 88 km, into Lake Champlain in western Vermont.