It has been shown that the force required to detach the holdfasts of Fucus vesiculosus depends upon the nature of the substratum; adhesion is less to barnacle shells. The lower adhesion appears to be due to a partial dissolution of the shell by acidic material, probably exudates. Growth of germlings on various substrata indicates that thalli and primary rhizoids develop better on calcareous substrata. Some ecological consequences of the phenomena are examined.