Littorina littorea was collected along a salinity gradient in the Scheldt estuary, located in the South of the Netherlands. Its morphological population structure was investigated to see whether salinity was correlated with shell size and shell weight. Shell size did not increase along the salinity gradient, as was expected, but rather showed a clear size transition between two salinity ranges (i.e., 10-20 ‰, and 21-30 ‰). Animals attain their largest size within a salinity range of 21-30 ‰. Relative shell weight did not vary consistently with salinity.