The development, settlement behaviour and settlement preferences of various marine tubeworm larvae were investigated in the laboratory and in the field. Scanning electron microscope observations of laboratory reared Pomatoceros lamarckii revealed several morphological features such as gland pores and ciliary tufts not reported previously. In still water laboratory assays using experimental slate surfaces and laboratory developed biofilms, P. lamarckii larvae required a bioflim for settlement and settled preferentially on older biofilms; settlement intensity was closely correlated to bacterial density suggesting that bacteria and/or their extracellular products are the inductive cue. The way in which experimental surfaces are presented to tubeworm larvae, either in single- or multi-treatment laboratory assays, significantly affected the settlement patterns of the spirorbids Spirorbis spirorbis and Spirorbis tridentatus but not P. lamarcidi, the larvae of which do not seem to become 'desperate' if not presented with a suitable surface for settlement. Interestingly, aerial drying a biofilm during simulated tidal emersion negates the settlement-inducing effect of a bioflim. S. spirorbis, S. tridentatus and Flustrellidra hispida (bryozoa) larvae also avoid recently dried biofilmed surfaces. Combined laboratory and field experiments showed that bioflims are an important settlement cue to P. lamarkii larvae, enabling them to settle selectively in their appropriate intertidal habitat. In the laboratory, monospecific bacterial films induced, had no effect upon or inhibited settlement of Spirorbis spirorbis larvae and experimental manipulation of biofilms suggested that the inductive bioflim cue is a commonly produced non-specific polysaccharide that is present either on the surface of the bacterial cell or in the extracellular polysaccharide matrix of the biofllm. The larvae of the tubeworms studied here display highly selective settlement preferences in response to biofllms and will continue to be useful subjects for further laboratory and field based studies that investigate how bioflims mediate the settlement of marine invertebrate larvae.