An 8-ha, extremely rich fen, surrounded by saline wet meadows, occurs on the western edge of mixed-grass prairie in the northwestern Great Plains, 47 km east of the Montana Rocky Mountain Front. The site occurs on an outwash plain near the junction of Pleistocene-age continental and mountain glaciers and on the western edge of former glacial Cut Bank Lake. Five wetland plant communities are described from this site. A total of 93 vascular plant and 65 diatom species have been found. Twelve diatom taxa could not be identified with available floras and one has been confirmed as new to science. Both diatoms and vascular plants indicate a locally disjunct, post-glacial assemblage similar to those found in boreal and mountainous habitats, adapted to alkaline waters. Saline-influenced, extremely rich fens are a rare sub-type of peatland in the northern Great Plains, and contain high diatom and vascular plant species richness as well as rare taxa.