The addition of ascorbate to aerobically growing cultures of Escherichia coli B caused only a short pause in growth and no subsequent change in the rate or extent of growth. The effect of ascorbate on oxygen uptake varied from inhibition in minimal medium to stimulation in rich medium. Cyanide-resistant growth and oxygen uptake were stimulated by ascorbate. Both the rate and extent of anaerobic growth were stimulated in proportion to the amount of ascorbate added when fumarate was the terminal electron acceptor. Ascorbate had no effect on any aspect of anaerobic growth in the absence of a terminal electron acceptor or in the presence of nitrate.