The amount of ATP required for the formation of microbial cells growing in a minimal medium with various nitrogen sources was calculated. In a glucose-mineral salts medium 28.8 g cells can be formed per mole ATP with ammonia and 23.1 for growth with nitrate. For growth with molecular nitrogen 11.1; 8.7; 7.1 and 6.0 g cells can be formed per mole ATP for ATP/N2 ratios of 12, 18, 24 and 30 respectively. A method is given for the calculation of Ysub, YO2 and Yc0 2 values for aerobic growth with glucose, succinate or methanol and various nitrogen sources. In this method use is made of the elementary composition of the cells and of mass balance equations. As an assimilation equation: C6H12O7 + 1.4 HNO3 + 6.85 “H2” → C6H10.84N1.4O3.07 + 8.13 H2O is given for growth ofParacoccus denitrificans with gluconate and nitrate. From this equation and the molar growth yield for gluconate the oxygen uptake, carbon dioxide evolution and the YO2 value can be calculated. A very good agreement between the calculated values and the experimental values was obtained. For the calculation of the ATP production it is essential to know the number of phosphorylation sites in the respiratory chain. Calculations are given for 2 (sites I + II) and 3 phosphorylation sites. The molar growth yields for growth with nitrate and nitrogen are much smaller than that for growth with ammonia. The YO2 values for growth with glucose and nitrate are higher (with 2 sites) or somewhat smaller (with 3 sites) than for growth with ammonia. The YO2 values for growth with nitrogen are always very low. The calculations show that especially YO2 is very dependent on the number of phosphorylation sites. For growth with methanol YCO 2 is strongly dependent on the nature of the assimilation pathway for methanol and on the nitrogen source. The molar growth yields for growth with glucose, succinate or methanol and nitrogen are about the same as when nitrate is the nitrogen source for organisms with 3 phosphorylation sites. The theoretical efficiency of nitrogen fixation in grow-ing cells is much lower (dependent on the ATP/N2 ratio) than that in nongrowing cells.