Serogroup 6 of Streptococcus pneumoniae contains three serotypes, named 6A, 6B and 6C, with highly homologous capsule gene loci. The 6A and 6B capsule gene loci consistently differ from each other by only one nucleotide in the wciP gene. The 6A capsule gene locus has a galactosyltransferase, which has been replaced with a glucosyltransferase in the 6C capsule gene locus. We considered that a new serotype named '6X1' would be possible if the galactosyltransferase of the 6B capsule gene locus is replaced with the glucosyltransferase of 6C. We demonstrate that this gene transfer yields a viable pneumococcal strain and that the capsular polysaccharide (PS) from this strain has the predicted chemical structure and serological similarity to the capsular PS of the 6B serotype. The new strain (i.e. serotype 6X1) is typed as 6B by the quellung reaction, but it can be distinguished from 6B strains with mAbs to 6B PS. Reexamination of 264 pneumococcal isolates that had been previously typed as 6B with classical typing methods revealed no isolates expressing serotype 6X1. Nevertheless, this study shows that this capsular PS is biochemically possible and could exist/emerge in nature. © 2009 SGM.