Nicotinamide adenine dinucleotide (NAD) and reduced NAD (NADH) levels have been measured in bacterial cultures. The cofactors were assayed by using the very sensitive cycling assay described previously by Cartier. Control experiments showed that the level of total NAD(H) falls during harvesting, and so samples were taken quickly from growing cultures and extracted immediately without separating the cells from the medium. Total NAD(H) ranged from 4.0 to 11.7 μmoles/g of dry cells for three facultative organisms, Klebsiella aerogenes, Escherichia coli , and Staphylococcus albus . NADH was remarkably constant in these bacteria; only one out of ten series of determinations was outside the range 1.4 to 1.9 μmoles/g of dry cells. NAD + showed much greater variation. An anaerobe ( Clostridium welchii ) had significantly more total NAD(H) whereas an aerobe Pseudomonas aeruginosa had about as much NAD(H) as the facultative organisms. NAD and NADH were measured during growth: once more NADH was much more constant than NAD. During change-over between aerobiosis and anaerobiosis, NADH showed a temporary increase but then returned to a constant level, whereas NAD changed from high aerobically to low anaerobically. These results are discussed in terms of the control mechanisms that may be involved.