Callitriche Stagnalis (Callitrichaceae) is a widespread aquatic species in the Old World, but an adventive component of the North American flora. Herbarium records are used to document the historical and current distribution of C. stagnalis in North America. It is hypothesized that the species initially established in North American seaports. The earliest collections of C. stagnalis in North America are from coastal regions in New York (1861), Oregon (1871), and New Jersey (1885). Prior to 1925, it is known from only a single noncoastal location (Montana, 1898). The rate of spread of C. stagnalis on the east versus west coast of North America differs. The species was well established on the east coast by the 1920s in New Jersey, New York, and southeastern Pennsylvania. On the west coast, only two locations (Oregon, British Columbia) were known by this time. The current distribution indicates that the species has spread about the same distance in both the east and west coastal regions (400-500 miles): west coast, from the first documented locality in Clatsop, Co., Oregon to Napa Co., California, and east coast from Mercer Co., New Jersey, to regions of the St. Lawrence in Quebec. Inland populations (Alabama, Montana, Wisconsin) are anomalous. Seed production is prolific in C. stagnalis and seeds are likely the principal unit of dispersal.