We report the occurrence of the rare and enigmatic diatom species Cymbopleura semirecta (Manguin ex Kociolek & Reviers) Bahls here for the first time from the southern Rocky Mountains. We review the current systematics and historic taxonomic placement of this taxon and its placement in both Cymbella and Cymbopleura. Valve morphology and ultrastructure are presented based on light (LM) and scanning electron microscope (SEM)_investigations. LM and SEM observations show this species lacks apical pore fields, a key feature helping to definitively place this taxon in Cymbopleura. We discuss the distribution of this species in higher latitude freshwater environments.