A parasitoid wasp larva is described emerging from an adult ant (Lasius sp.) in Eocene Baltic amber. On the basis of the cephalic structures of the parasitoid, it can be assigned to the subfamily Neoneurinae of the family Braconidae. This is the only extant subfamily of Hymenoptera whose females oviposit in adult worker ants. This discovery, which is the first fossil demonstrating insect parasitism of ants, shows that ant parasitism by Braconidae was well established some 40 million years ago. The parasitoid could belong to the extinct neoneurine, Elasmosomites primordialis Brues, which was described earlier from Baltic amber.