Estuarine Fucus vesiculosus from Hilbre Island R. Dee, Merseyside, has been found to produce receptacles of significantly greater dry weight than non-estuarine plants. The Hilbre plants also produced a greater biomass of reproductive than vegetative thallus. Comparable data from plants on sheltered and exposed marine shores suggest that inter-population differences in relative production of reproductive and vegetative tissues may be adaptive to habitat conditions. © 1979.