The fatty acid composition of total cellular lipids and lipid A of lipopolysaccharides was studied in Providencia alcalifaciens and P. stuartii. Significant differences were found in the fatty acid composition of total cellular lipids from the above species; these are caused mainly by the presence of cyclopropane fatty acids (methylenehexadecanoic and methyleneoctadecanoic acids) in P. stuartii and their absence in P. alcalifaciens. Differences were also established in the content of fatty acids in lipid A of lipopolysaccharides from P. alcalifaciens and P. stuartii. The data are consistent with the idea that P. alcalifaciens and P. stuartii are separate taxons though, apparently, closely related.