SUMMARY1. The degree to which fish faunas have been invaded by exotic species is quantified on the basis of river basins.2. Nineteen exotic species are now established in Spanish fresh waters.Alburnus alburnus(L., 1758) is reported here for the first time.3. The native/total fish ratio varies from 0.71 in Galicia, the best preserved fish fauna, to 0.41 in Douro, the most modified fish assemblage. The national average ratio (0.63) is similar to that reported for Portugal (0.65), but higher than that in Italy (0.56) and much lower than that in Greece (0.88).4. Exotic fishes have sometimes become successfully established over wide areas in a short time.