Lactobacillus delbrueckii subsp. bulgaricus ATCC 11842 was grown in a chemostat at 45 degrees C and pH5.5 using glucose as the carbon source, with the aim of optimizing biomass production. Cells were grown in a complex medium under nitrogen. At dilution rates lower than 0.18 h(-1), it was difficult to keep steady-state conditions and pleomorphic forms were observed. The addition of 30mM Ca2+ and Mn2+ reverted the cells to normal shape : 30mM Mg2+ had no effect. Increasing the dilution rate resulted in normal morphology without the addition of any cations. Under these conditions, a maximum productivity of 1.24g dry biomass 1(-1) h(-1) was obtained. The maximum growth yield, corrected for maintenance, was 30g biomass mol(-1) glucose and the maintenance energy was 0.26g glucose g(-1) biomass h(-1). Lactate was the main fermentation product at all glucose concentrations used in the feed medium. Cells grown at high dilution rates had normal technological properties (acid production and proteolysis) when tested in milk.