1. The influence of the gut microflora on lipid metabolism was investigated in germ-free (GF) and conventional (CV) laying Japanese quail. 2. Serum and egg yolk cholesterol concentrations showed comparable values in both GF and CV environments. 3. The fatty acid composition of liver lipids was modified by the presence of gut microflora. Notably, in the presence of the gut microflora, proportion of oleic acid was reduced and conversely, stearic and linoleic acids were enhanced. 4. In egg yolk lipids, the proportion of myristoleic and palmitoleic acids was significantly lowered and that of stearic acid was significantly enhanced by the presence of the gut microflora, though the difference was very small. 5. It was suggested that oleic acid could be easily either hydrogenated to stearic acid or desaturated to linoleic acid by the action of the gut microflora in Japanese quail.