A new species of alderfly (Megaloptera: Sialidae) is described and figured from a rare adult specimen preserved in middle Eocene (Lutetian) Baltic amber. Sialis (Protosialis) voigti, new species, is distinguished from the two other species of Protosialis recorded from Baltic amber. The Megaloptera documented from Baltic amber are briefly overviewed and the geological history of Sialidae summarized.
