Variation in esterase expression (EST) has been investigated in 360 specimens of the periwinkle Littorina littorea, collected at nine sites along the polluted western Scheldt and the relatively clean eastern Scheldt estuary. Mean number of EST bands differed among the sites, while a Dice similarity based multi-dimensional scaling along with a minimum spanning tree procedure, revealed an estuary based structuring, clustering the most polluted and least saline like sites together. The underlying factors and mechanisms (i.e. selection, regulation) which are responsible for this structuring remain to be determined © 2001 Elsevier Science Ltd. All rights reserved.