The fatty acid compositions of forty-five strains of anaerobic curved rods isolated from vaginal content were analysed by gas chromatography. Twenty-six of the strains were of a short Gram-variable type and 19 of a long Gram-negative type. The bacteria were subjected to alkaline saponification, after which the liberated fatty acids were methylated using boron trichloride-methanol. The samples were introduced onto a cross-linked (SE-54) fused silica capillary column. The major peaks in the chromatograms from all strains corresponded to C14:0, C16:0, C18:2, C18:1 and C18:0 fatty acids. Five additional components were seen, three of them identified by mass spectrometry as fatty aldehydes (saturated and unsaturated) of 16 to 18 carbon atoms, whereas the remaining two were not identified. The fatty aldehydes indicated the presence of plasmalogens in the bacterial strains studied. With two exceptions, all of the short rods were distinguishable from the long ones by the presence of one aldehyde and two unidentified compounds, all three eluting between C16:0 and C18:2.