An investigation of certain aspects of the biology of Pomatoceros lamarckii and Laminaria digitata was carried out with particular reference to the factors influencing the settlement of P. lamarckii larvae and to the bioadhesion of both species. The larvae of P. lamarckii were found to be negatively phototaxic throughout their development. A distinctive pattern of settlement behaviour was observed and as the larvae settled they became attached to the substratum by a mucus pad situated in the region of the ventral shield epithelium. Larvae settled gregariously on and close to conspecific adults, but the presence of conspecific juveniles did not induce settlement. Unsuccessful attempts were made, using whole and parts of animals and tubes in a range of solvents, to isolate a chemical settlement stimulus and it was concluded that it is highly unlikely that a chemical cue is produced by P. lamarckii adults. Biofilming was found to be the major factor in inducing settlement and it was demonstrated that substrata biofilmed in the presence of P. lamarckii adults attracted significantly more settlers than did substrata biofilmed in other ways. The adult mediated biofilms were found to contain large numbers of a rod-shaped bacterium which was present only in very low numbers in the other biofilms and it is proposed that this bacterium, or its exopolymers, may provide the primary settlement stimulus. Natural settlement was examined on pebbles at three different locations and the results compared with the laboratory findings. Tenacity was measured for the tubes of P. lamarckii and the haptera of L. digitata. A mean tenacity of 24.75 x 1()5 ± 6.7 N m-2 was recorded for P. lamarckii and a mean tenacity of 4.21 x 1()5 ± 1.9 N m-2 was recorded for L. digitata.