The O polysaccharide (OPS) of the lipopolysaccharide (LPS) of Pseudomonas syringae pv. atrofaciens IMV 7836 and some other strains that are classified in serogroup O1 was shown to be a novel linear -D-rhamnan with the tetrasaccharide O repeat 3)--D-Rhap-(13)--D-Rhap-(12)--D-Rhap-(12)--D-Rhap-(1 (chemotype 1A). The same -D-rhamnan serves as the backbone in branched OPSs with lateral (13)-linked D-Rhap, (14)-linked D-GlcpNAc, and (14)-linked D-Fucf residues (chemotypes 1B, 1C, and 1D, respectively). Strains of chemotype 1C demonstrated variations resulting in a decrease of the degree of substitution of the backbone 1A with the lateral D-GlcNAc residue (chemotype 1C-1A), which may be described as branched regular  branched irregular  linear OPS structure alterations (1C1C-1A  1A). Based on serological data, chemotype 1D was suggested to undergo a 1D  1D-1A alteration, whereas chemotype 1B showed no alteration. A number of OPS backbone-specific monoclonal antibodies (MAbs), Ps(1-2)a, Ps(1-2)a₁, Ps1a, Ps1a₁, and Ps1a₂, as well as MAbs Ps1b, Ps1c, Ps1c₁, Ps1d, Ps(1-2)d, and Ps(1-2)d₁ specific to epitopes related to the lateral sugar substituents of the OPSs, were produced against P. syringae serogroup O1 strains. By using MAbs, some specific epitopes were inferred, serogroup O1 strains were serotyped in more detail, and thus, the serological classification scheme of P. syringae was improved. Screening with MAbs of about 800 strains representing all 56 known P. syringae pathovars showed that the strains classified in serogroup O1 were found among 15 pathovars and the strains with the linear OPSs of chemotype 1A were found among 9 of the 15 pathovars. A possible role for the LPS of P. syringae and related pseudomonads as a phylogenetic marker is discussed.