The chromosomes of 346 specimens ofMytilus galloprovincialis, collected at four NW Iberian peninsula sites, were studied using a C-banding technique. Polymorphism for C-banding was detected. According to the banding patterns, three classes of mussels could be distinguished: 1) individuals showing two pairs of submeta/subtelocentric chromosomes bearing NOR-associated heterochromatic regions terminal to their long arms, 2) mussels presenting additional C-bands located at other positions on the NOR-bearing chromosomes or on other chromosomes, and 3) mosaic mussels. The location of the additional C-band positive regions was similar in the four populations studied.