The presence of M. galloprovincialis-type genes among the population of mussels in the Dutch Wadden Sea, historically described as M edulis, was assessed. We applied the molecular technique in which a fragment of the gene coding for an adhesive protein of the byssus of mussels is amplified by PCR and assayed for length using electrophoresis. Among 321 individual mussels collected in August-October 2001 at 14 sites (5 intertidal, 9 subtidal) widely dispersed over the Dutch Wadden Sea, 6 specimens (collected at 5 sites) were found that showed a heterozygote genotype with both the M edulis- and the M galloprovincialis-type alleles being amplified; all others were identified as homozygotes for the M edulis-type allele. Differentiation in frequencies of heterozygotes among sites was not detected. The ofact that the M galloprovincialis-type allele was present at low frequency (0.0093) may be attributed to one of three possible, and not mutually exclusive, causes: incomplete diagnosticity of this marker, an historically stable introgression zone in the Wadden Sea, or a recent invasion. (C) 2002 Elsevier Science B.V. All rights reserved.