Poison frogs of the neotropical family Dendrobatidae contain a wide variety of lipophilic alkaloids, which are accumulated from alkaloid-containing arthropods. A small millipede, Rhinotus purpureus (Siphonotidae), occurs microsympatrically with the dendrobatid frog Dendrobates pumilio on Isla Bastimentos, Bocas del Toro Province, Panamá. Methanol extracts of this millipede contain the spiropyrrolizidine O-methyloxime 236, an alkaloid previously known only from skin extracts of poison frogs, including populations of D. pumilio. Thus, R. purpureus represents a likely dietary source of such alkaloids in dendrobatid frogs.