tVe measured quantitative variation of structurally siinllar halogenated furanones in the sea hare Aplysia parvula (Opisthobranchia; Anaspidea) and in its host alga Delisea pulchra (Rhodophyta; Bonnemaisonales). Mean total levels of furanones from D. pulchra in A. parvula were 13.3% of the dry weight of the sea hares, with one metabolite comprising on average 86% of the total metabolite load of the sea hares. Levels of furanones in the sea hares were highest in the digestive gland but were also found in other tissues, including the skin in at least mg g -' (dry wt) levels. Metabolite levels in the skin of the sea hares did not differ from those in D. pulchra (typically between 5 and 10 mg g.' dry wt). Variation of metabolites in D. pulchra on the scale of both meters and kilometers was low and only minor variation in levels of individual metabolites was observed. There was significant variation in levels of metabolites within plants, with concentrations generally higher at the distal end of the thallus. The large differences in metabolite concentrations on a scale of millimetres found in some other red algae were not observed in D, pulchra. Metabolites occurred in significantly different relative amounts in A. parvula versus D pulchra, with concentrations of ind~v~dual metabolites in A. parvula ranging between 0 and 83 times the concentrations found in host algae. These results show that the sea hares differentially bioaccumulate algal metabolites. However, only 1 metabolite was enhanced in concentration in the sea hares relative to the alga. This metabolite was the most abundant compound in the animals, and has been previously shown to be effective as a predator deterrent. A second metabolite, which was not effective as a predator deterrent in A. parvula, decreased in relative concentration in the animals. In contrast to recent suggestions in the literature for sequestered algal metabolites in sea hares, our results indicate that the distribution and level of D. pulchra metabolites in A. parvula are consistent with a role as acquired chemical defences against predators.