A fossil mermithid nematode, Heydenius brownii sp. n. (Nematoda: Mermithidae) emerging from a planthopper (Hemiptera: Fulgoroidea: Achilidae) in Baltic amber, is described. This is the first parasite of a fossil member of the order Hemiptera. There are no reports of mermithid parasitism of extant achilids; however, other families of planthoppers are attacked by mermithids in North America, Asia and Australia today. This find shows that parasitism of achilid fulgorids by mermithid nematodes was well established some 40 million years ago.