The sacoglossan mollusc-derived metabolite, tridachiahydropyrone (3), and its proposed biosynthetic precursors (1 and 2) form part of a complex chemical defence system against predators and harmful UV light. Here, we provide supporting biophysical evidence that the metabolites become selectively localised at cell membranes and outline a binding scheme that accommodates the observed data. The possibility that localised lipid domains within the membrane have an effect on the localisation is also addressed.