Vitamin K depletion in the vitamin K-requiring anaerobe Fusiformis nigrescens resulted in the formation of elongated cells. In the presence of vitamin K, inhibition by cyanide and heme depletion did not affect cell morphology. Peptide depletion, as well as inhibition by chloramphenicol, ethionine, or fluorophenylalanine, also resulted in formation of elongated cells. Chemical analysis of cells and envelopes showed that protein, lipid, and deoxyribonucleic acid levels were similar in vitamin-supplemented and depleted cells; however, ribonucleic acid (RNA) levels in depleted cells and envelopes were significantly higher than in K-supplemented cells. Sections of cells of both groups showed minor differences in cell surfaces. The relationship between altered cell envelopes and increased RNA levels in K-deficient bacteria indicated a membrane-mediated effect of vitamin K on RNA and possibly protein synthesis in this microorganism.