Escherichia coli was grown as a continuous culture at various defined conditions of temperature, pH, aeration rate and dilution rate. The lipids were extracted from disrupted cells and the relative fatty acid content of the individual and total phospholipids was determined. The lipid composition of E. coli was shown to change with the fermentation conditions. Interestingly, E. coli adapted to high growth rates and to low oxygen tension by changing the lipid composition of the membrane in exactly the same way, thus indicating a common effect.