The growth energetics of a typical lactic acid bacterium Lactococcus lactis subspecies cremoris FD1 has been studied in (1) batch, fed-batch and continuous cultures; (2) complex and defined media; (3) at energy, carbon and N-source limitation and (4) using the following sugars as energy source : glucose, fructose, galactose and lactose. The growth energetics is described by two parameters : the growth associated ATP consumption, Y-xATP (mmol ATP g(-1) biomass), and the non-growth associated ATP consumption, k(ATP) [(mmol ATP)(g biomass)(-1) h(-1)]. The theoretical ATP requirement for biomass synthesis is estimated to Y-xATP(growth) = 29 mmol g(-1) and this is the theoretical minimum of Y-xATP. Whereas values of Y-xATP considerably larger than Y-xATP(growth) obtained whenever the medium is deficient in any of the necessary growth substrates it is surprising that values of Y-xATP much smaller than Y-xATP(growth) can also be obtained, namely for growth on yeast extract at conditions where the growth reactions are neither limited by the carbon-, the nitrogen- or the energy source. The low Y-xATP is tentatively ascribed to consumption of peptides at the beginning of a batch fermentation or at high dilution rates in a chemostat. In a defined medium Y-xATP is close to the theoretical value until certain essential amino acids are used up. After that Y-xATP increases to 70-120 mmol g(-1). The same high values of Y-xATP are found when biomass synthesis is limited by lack of energy source. When biomasss synthesis is limited by synthesis of certain biomass precursors (glucose-6-phosphate or fructose-6-phosphate) Y-xATP increases dramatically to 70-330 mmol g(-1).