A female stylopid parasitising an ant of the genus Prionomyrmex (Hymenoptera: Formicidae) is described and illustrated from Baltic amber (Eocene, 39-50 million years old). This new fossil is tentatively considered as a plesiomorphic member of the Myrmecolacidae. Recent Myrmecolacidae parasitise dual hosts: males parasitise Formicidae, while females parasitise either Mantodea or Ensifera. This is the first record of a female stylopid parasitising Formicidae. The phylogenetic relationships of the Myrmecolacidae and the origin of their unique dual host relationship are discussed.