Methylococcus capsulatus contained extensive intracytoplasmic membranes when grown in fed-batch cultures over a wide range of oxygen tensions (0.1 to 10.6%, vol/vol) and at a constant methane level. Although the biomass decreased as oxygen levels were lowered, consistently high amounts of phospholipid and methyl sterol were synthesized. The greatest amounts of sterol and phospholipid were found in cells grown between 0.5 and 1.1% oxygen (7.2 and 203 mumol/g [dry weight], respectively). While sterol was still synthesized in significant amounts in cells grown at 0.1% oxygen, the major sterol product was the dimethyl form. Analysis by capillary gas chromatography-mass spectrophotometry showed that the phospholipid esterified fatty acids were predominantly 16:0 and 16:1 and that the hexadecenoates consisted of cis delta 9, delta 10, and delta 11 isomers. At low oxygen tensions, the presence of large amounts (25%) of cyclopropane fatty acids (cy 17:0) with the methylene groups at the delta 9, delta 10, and delta 11 positions was detected. Although the delta 9 monoenoic isomer was predominant, growth at low oxygen levels enhanced the synthesis of the delta 10 isomers of 16:1 and cy 17:0. As the oxygen level was increased, the amount of cyclopropanes decreased, such that only a trace of cy 17:0 could be detected in cells grown at 10.6% oxygen. Although M. capsulatus grew at very low oxygen tensions, this growth was accompanied by changes in the membrane lipids.