The genera Microplitis and Snellenius (Hymenoptera: Braconidae, Microgastrinae) from Area de Conservacion Guanacaste (ACG), Costa Rica, are revised. A total of 28 new species are described: 23 of Snellenius (the first record for Mesoamerica) and five of Microplitis. A key is provided to all new species and five species of Microplitis previously described from Mesoamerica. In ACG, all Microplitis were reared exclusively from Sphingidae, while all Snellenius were reared from Noctuoidea (Noctuidae and Erebidae). All of the wasp species with known host records are unambiguously specialists, parasitizing one or a few related hosts. Biological information (wasp cocoon and caterpillar hosts) in the Neotropical region seems to differ from similar data reported in previous works for the Oriental region -but more studies on the world fauna are needed. Although the distinction between these two genera has been controversial, we consider that the available evidence, although not conclusive, suggests that these two genera are best kept as separate (based on the presence of at least a partial epicnemial carina in Snellenius, which is absent in Microplitis). The following 28 species, all authored by Fernández-Triana & Whitfield, are described as species nova: Microplitis adrianguadamuzi, M. alexanderrojasi, M. francopupulini, M. hebertbakeri, M. jorgehernandezi, Snellenius billburgeri, S. bobdressleri, S. donstonei, S. felipechavarriai, S. gerardoherrerai, S. irenebakerae, S. isidrochaconi, S. johnkressi, S. jorgecampabadali, S. jorgegomezlauritoi, S. josesarukhani, S. kerrydresslerae, S. lucindamcdadeae, S. luisdiegogomezi, S. mariakuzminae, S. mariamartachavarriae, S. phildevriesi, S. quiricojimenezi, S. robertoespinozai, S. sandyknappae, S. velvaruddae, S. vickifunkae, S. warrenwagneri.