The free-living flatworm clade Macrostomorpha is historically understudied, yet contains a novel invertebrate model organism, Macrostomum lignano, which is used for studies of, e.g. stem cells, regeneration, bioadhesion and reproduction. Additionally, Macrostomorpha has the potential to become an extended testing-ground for hypotheses of sexual conflict in hermaphrodites, which are currently being studied by Dr. Lukas Schärer (the host of this application) within the genus Macrostomum. Although the first molecular phylogeny for Macrostomorpha has provided a clarified picture of morphological evolution in this group, further work in this area is currently hindered by the large number of species that have languished undescribed for decades (many were intended to be published the third part of a monographic treatment of Dolichomacrostomida by the late Prof. Rieger, University of Innsbruck). Moreover, there is a paucity of described species from the western Atlantic, especially in the basal clades Haplopharyngidae, Myozona, Bradynectes and among some Dolichomacrostomidae. Previous work by Prof. Julian Smith (the guest this application) while visiting Innsbruck has recovered much of the unpublished material from Rieger’s intended monograph, and he has supplemented this material with new collections from the Western Atlantic (continental U.S. and Tobago). Taken together, this material comprises 16 species from the four basal clades mentioned above. We propose to publish detailed taxonomic and anatomical descriptions these species based on serial resin sections, sequence four genes from each of 13 of these species, and subsequently refine the current multi-gene phylogeny of Macrostomorpha using these new data. The impact of this work will be both immediate (i.e. a more complete picture of the biodiversity and morphological evolution in Macrostomorpha) and lasting (i.e. further collaborations on the taxonomy of Macrostomorpha between the Schärer and Smith labs).