The Gilmore City Formation of north-central Iowa represents an upward-shoaling sequence of carbonate rocks ranging from normal marine conditions in the lower portion of the section to prograding oolite shoals at the top. At most exposures, the formation contains an abundant and diverse fauna that has been used in several attempts to correlate the Gilmore City with the Mississippian type section. Results of biostratigraphic correlation indicate that the Gilmore City Formation spans the Kinderhook-Osage boundary, occupying a position that, in the Mississippian type section, is represented by an unconformity. Gilmore City fossils actually have closer affinities with Cordilleran faunas than with those of the Midcontinent.The gastropods constitute one of the most abundant and diverse faunal elements of the Gilmore City Formation, consisting of more than 75 species representing about 50 genera and subgenera. This first part deals only with the Bellerophontoidea and Euomphaloidea. Described and illustrated species include the bellerophontoideans Bellerophon (Bellerophon) panneus White, 1862, Bellerophon (Bellerophon) meeki? Koninck, 1883, Waagenella spergenensis (Gordon and Yochelson, 1983), Waagenella sp. cf. Waagenella spergenensis (Gordon and Yochelson, 1983), Waagenella sp. indet., Retispira sp. cf. Retispira exilis (Koninck, 1883), and Euphemites rollinsi, new species; and the euomphaloideans Euomphalus springvalensis White, 1877a, Euomphalus luxus White, 1877b, Euomphalus ammon White and Whitfield, 1862, Straparollus obtectus, new species, and Serpulospira paradoxus (Winchell, 1864).