Formate stimulates growth of a new bacterium from human feces. With high formate, it ferments glucose to acetate via the Wood-Ljungdahl pathway. The original isolate fermented vegetable cellulose and carboxymethylcellulose, but it lost this ability after storage at −76°C. 16S rRNA gene sequencing identifies it as a distinct line within the Clostridium coccoides supra-generic rRNA grouping. We propose naming it Bryantella formatexigens gen. nov., sp. nov.