Two of the 13 genera of the subfamily Geocorinae are present in the study area: Isthmocoris with 2 species and Geocoris with 14 species. These genera and 15 of the species are redescribed and G. barberi n. sp. Is described. Geocoris sonoraensis Van Duzee, G. pallens solutus Montandon, G. guatemaltecus Torre-Bueno, G. ullginosus lateralis Fieber, and G. ullginosus speculator Montandon are reduced to synonymy and G. bullatus floridanus Blatchley, G. ullginosus limbatus Stal, and G. ullginosus howardi Montandon are raised to species status. Neotypes are designated for G. punctipes (Say), G. ullginosus (Say), G. bullatus (Say), and I. piceus (Say).

The genera Isthmocoris and Geocoris are redescribed. Isthmocoris McAtee is raised from synonymy with Hypogeocoris Montandon. Three subgroups within the genus Geocoris are recognized based largely on differences in head sulcation and comparison is made between these 3 subgroups and the European species G. grylloides (L.), the type-species of the genus Geocoris.

Ecological and biological data are presented for each species and distribution records are mapped by counties. Interspecific mating studies in the laboratory among 10 species showed strong pre-mating reproductive Isolation.

Keys are presented for the geocorine genera, to the species of the genus Isthmocoris, and to the species of Geocoris within the study area. The genitalia are dissected and the spermathecae and parameres are drawn. Dorsal view Illustrations are made for each species.