An arenaceous Foraminifera fauna occurs within spongiostromatid algal oncolites which are part of a thin biostromal calcareous algal-sponge nodular zone in the lower part of the early Mississippian Kinderhookian Sappington Formation. The fauna is extracted from hydrochloric acid residues of oncolites from 7 localities in southwestern Montana. Seven genera and 10 species are described, including Oxinoxis, n. gen., and Oxinoxis botrys, n. sp., Hyperammina sappingtonensis, n. sp., and Tolypammina continuus, n. sp. This benthonic biota with epifauna on nodular calcareous algae is part of an intertidal bank covered with shallow agitated water exposed to mild wave action. The nodular substratum of the biostrome serves as a sediment trap for protection of the adnate fauna of corals, bryozoans, brachiopods, worm tubes, and the wholly arenaceous Foraminifera fauna. The Foraminifera are closely related to other early Mississippian faunas and are similar, if not identical to those in the Louisiana Limestone of Missouri.