| Summary: | A new species, <i>Aulacoseira wyomingensis</i>, is described from a small seep near Casper, Wyoming, USA, using light and scanning electron microscopy, in which it was the dominant diatom taxon. Valves have large areolae that cover the valve face. Spines, when present, are short and shield-like, precluding chain formation. Unlike most members of the genus, <i>A. wyomingensis</i> appears to lack rimoportulae. While we assign this species to the genus <i>Aulacoseira</i> within the Aulacoseiraceae based on the radial nature of its valve, its cingulum composed of fine ligulate bands, and presence of a ringleiste and collum, it does not appear to be easily assigned to any of the morphological groups within the genus.
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