The thesis describes experiments investigating interactions between barnacle cypris larvae and bacterial films covering substrata. The work begins with a confirmation of the link between temporary adhesion of cypris larvae and subsequent settlement. The following chapters then investigate, in detail, barnacle/biofilm interactions by measuring cypris temporary adhesion to films. Initially, bacterial strains were isolated from solid surfaces in the intertidal areas of the Menai Strait in an attempt to correlate bacterial zonation on the shore with barnacle zonation. However, the results of bacterial collection were inconclusive and a more in-depth study would have proved too onerous to be completed in this body of work. Instead the bacterial strains isolated were used to study the effect of monospecific films and their products on the larvae of Verruca strdemia larvae. The results show that individual biofilms differ in their likelihood to encourage settlement and that exopolysaccharides from one of the isolates (Pseudomonas W1+) have a concentration dependent inhibitory effect upon temporary adhesion. The next chapter deals with the effects of shear upon biofilms and the subsequent effect upon the temporary adhesion of Elminius modestus and Balanus perforates cyprids. The work demonstrates that the two species may appreciate differences in bacterial communities grown under contrasting shear regimes. Finally, the effect of individual polysaccharide components of bacterial films upon temporary adhesion of five barnacle species is studied showing that monosaccharides have an inhibitory effect upon temporary adhesion, but that pentoses have the least effect, whilst hexoses have a more marked effect. The greatest inhibition is caused by uronic acids, probably because of their more polar nature. The overall conclusions suggest that the physico-chemical nature of bacterial films has a more profound effect upon cypris adhesion and settlement than species composition and that bacterial films do have a role to play in settlement and possibly in zonation of barnacles.