Mesocrina chandleri Godfray & van Achterberg, sp. nov., is described in the small Holarctic genus Mesocrina Foerster, 1863, the second European species. The holotype was collected in England and further specimens are recorded from Finland, France and the Netherlands. A key is provided to the Palaearctic Mesocrina species. DNA sequence from the CO1 barcode locus was obtained and the new species is 10% divergent from M. indagatrix (the other European species) and 5% divergent from an undetermined North American species. We provide evidence that Mesocrina spp. parasitise cyclorrhaphan Diptera in fungi (and that previous host records from phytophagous insects are incorrect) and that the taxon is not part of the Dapsilarthra genus-group as often previously assumed.