The distribution and population structure of C. fluminea in Britain was investigated during February 2000. The presence of large individuals (32 mm length) and absence of any dead shells suggests that C. fluminea arrived in 1997. It now occurs in East Anglia at four sites within a 12 km radius. The greatest densities are in the River Chet, Norfolk, (up to 2500 individuals m²) where the population is dominated by small individuals (2 to 9 mm in length). The potential impacts that C. fluminea may have on Britain’s economy and ecology are discussed.