A previously undescribed myxobacterium was isolated from marine mud and studied in pure culture. This organism is a typical non-fruiting, amicro-cystogenous lower myxobacterium and has been classified in the genus Cytophaga under the name C. fermentans, n.sp., to indicate its ability, unique among the cytophagas so far studied, to grow anaerobically at the expense of organic compounds. The organism can utilize as carbon and energy sources simple sugars and starch, but not cellulose, agar or chitin. Ammonium salts, asparagine or glutamine are satisfactory nitrogen sources; urea, nitrates and amino acids are not. Thiamine and relatively high concentrations of iron and CO2 are required for growth in a defined medium. Glucose is fermented to acetic, propionic and succinic acids, in approximately equimolar quantities, plus a small amount of lactic acid. In addition, a large quantity of mucilage, which appears to be a nitrogen-containing substance, is produced during the fermentation.