Free-swimming planktonic larvae of most species of sedentary marine invertebrates do not metamorphose in mid-water and settle at random, but possess considerable powers of delaying metamorphoses until a suitable substrate is encountered. Within recent years several workers have analysed the physical properties of a substrate to which larvae respond, particularly amongst those species which are restricted in their distribution in the littoral zone to the surfaces of other animals and plants. The serpulid Spirorbis borealis occurs most abundantly on Fucus serratus; experiments conducted in which surfaces treated with extracts of F. serratus were offered as alternatives to untreated surfaces illustrated that the larvae settle on these extract-treated surfaces in significantly greater numbers. It has been demonstrated that this response is to some chemical substance derived from the alga rather than to changes in physical properties of the surface such as increased thickness of the overlying film, or changes in the surface texture. Contact with this substance modifies the settlement behaviour from that observed amongst larvae settling on untreated surfaces. The photopositive behaviour is curtailed, and the duration of the crawling behaviour preceding settlement abbreviated. The precise mechanism by which the larvae respond to the active substance is as yet unknown, but observations have illustrated that it is dependent upon the larvae detecting the active substance as an adsorbed layer on the surface rather than a response to concentration gradients in solution. The significance of an innate ability to detect specific substrates by chemical structures characteristic of the surface in the speciation of the Spirorbinae has been discussed.