Core oligosaccharides were released by mild acid hydrolysis of the lipopolysaccharides of 25 strains of bacteria Citrobacter representing 19 O-serogroups. Screening by sugar analysis and high-resolution ESI MS revealed structural heterogeneity of the core within each strain and core structure variations between strains. The core oligosaccharides from Citrobacter werkmanii O14 strains PCM 1548 and 1549 were further purified by anion-exchange chromatography and their full structures were elucidated by methylation analysis and two-dimensional NMR spectroscopy; similar inner core regions and significantly different outer core pentasaccharide regions were established. D-Galacturonic acid 3-(2-aminoethyl)phosphate was found to be a characteristic component of the Citrobacter core.