This paper presents 13 new records of Laboulbenialean parasites on ground beetles (Coleoptera, Carabidae) from the Boston Harbor Islands National Recreation Area in Massachusetts: Laboulbenia anoplogenii, L. casnoniae, L. clivinalis, L. egens, L. filifera, L. flagellata, L. inflata, L. macrotheca, L. pedicellata, L. terminalis, L. variabilis, L. vulgaris, and Peyritschiella geminata. Laboulbenia clivinalis and L. egens are new country records for the US. Moreover, we present additional localities for L. casnoniae, L. clivinalis, L. filifera, L. flagellata, L. inflata, L. pedicellata, L. variabilis, and L. vulgaris. The following new country records are presented: Laboulbenia clivinalis, L. filifera, and L. variabilis from Canada; L. flagellata from the Democratic Republic of the Congo; L. pedicellata from Ukraine; L. vulgaris from Croatia and Slovenia (and the first undoubtful record from Slovakia). Laboulbenia flagellata was found on 11 host species in the genera Agonum, Oxypselaphus, Patrobus, Platynus, and Pterostichus. Using this abundant material, we performed morphometrics to test the hypothesis that L. flagellata is a species complex. Specimens cannot be separated based on host genus (Agonum, Pterostichus). One parameter is significant between Pterostichus mutus and each of the 4 Agonum species after applying a strong Bonferroni P-value correction: H1T, the ratio of height of cell I (HC1) to total thallus length (TTL). In addition, we collected fresh material to be able to add a molecular phylogenetic component to test said hypothesis. We generated ITS and nrLSU ribosomal sequences of several species of Laboulbenia, including isolates of L. flagellata from multiple hosts. Phylogenetic inference of the concatenated dataset shows that L. flagellata isolates from 3 host species form 2 distinct clades, providing support for our hypothesis. We also show that L. coneglianensis is separate from L. flagellata, unequivocally ending a long-standing taxonomic debate. Finally, examination of Roland Thaxter's 1891–1932 slides led to the designation of lectotypes for L. macrothecia, L. terminalis, and P. geminata.