1. The energetics of Escherichia coli W growing aerobically in continuous culture have been investigated. Conditions were chosen such that growth was limited by the availability of carbon or oxygen (energy-limited cultures), or of ammonium of sulphate ions (excess energy cultures). 2. Under glycerol-limited conditions YmaxO2 (true molar growth yield with respect to oxygen) and YmaxATP (true molar growth yield with respect to ATP equivalents) were 50.9 g cells-mol O-02(-1) and 12.7 g cells-mol ATP equivalents-1 respectively; these values were not substantially altered during growth limited by oxygen, ammonium or sulphate. In contrast, M (the energy requirement for maintenance purposes) increased from approximately 2 mmol ATP equivalents-h-1-g cells-1 during energy-limited growth to 16.8 and 30.8 mmole ATP equivalents-h-1-g cells-1 when growth was limited by ammonium and sulphate ions respectively. 3. Replacement of glycerol by other limiting carbon sources caused YmaxATP to alter within the range 13.9 (glucose) to 7.1 (acetate) g cells-mol ATP equivalents-1 in the order glucose greater than galactose greather than arabinose greater than fructose greater than glycerol greater than fumarate greater than lactate greater than pyruvate greater than acetate. In each case the experimental value of YmaxATP was less than or equal to 55% of the theoretical value calculated from the known energy requirements for the biosynthesis of cell materials. 4. It is concluded from these results that neither M nor Ymax ATP are constant values for E. coli. M varies with the energy supply, being highest under excess energy growth conditions where it may reflect energy wastage by the cell. On the other hand, YmaxATP varies with the nature of the growth substrate and thus reflects the different energy requirements for the synthesis of cell material from different carbon sources.