Cells of Escherichia coli contained an altered phospholipid and fatty acid composition when grown in the presence of some pesticides. Whereas parathion increased the concentration of all phospholipid species without changes in their polar head groups, DDT (dichlorodiphenyltrichloroethane) decreased the proportion of neutral serine-derived phosphatides and dieldrin decreased the proportion of negatively charged phospholipids. The saturated/unsaturated plus cyclopropane fatty acid ratio was increased in all cases. The changes suggested that cells adapted their membrane lipids to compensate for the presence of pesticides in the environment.