Abstract Growth of Methylococcus capsulatus (Bath) at temperatures ranging from 30 to 50°C resulted in changes to the whole cell lipid constitutents. As temperature was lowered, the overall proportion of hexadecenoic acid (C16:1) increased, and the relative proportions of the Δ9, Δ10 and Δ11 C16:1 double bond positional isomers changed. Methyl sterol content also increased as the growth temperature was lowered. The highest amounts of methyl sterol were found in 30°C cells and the lowest in 50°C cells (sterol-phospholipid ratios of 0.077 and 0.013, respectively). The data are consistent with a membrane modulating role for the sterol produced by this prokaryotic organism.