The genetic basis for biosynthesis of the (α1→4)-linked N-acetyl-d-glucosamine 1-phosphate capsule of Neisseria meningitidis serogroup X was defined. The biosynthesis gene cassette was a ∼4.2-kb region located between ctrA of the capsule transport operon and galE, which encodes the UDP-glucose-4-epimerase. This location was identical to the locations of the biosynthesis cassettes in other meningococcal serogroups. Three open reading frames unique to meningococcus serogroup X were identified. Deletion-insertion mutation and colony immunoblotting confirmed that these three genes were essential for serogroup X capsule expression, and the genes were designated xcbA, xcbB, and xcbC (serogroup X capsule biosynthesis). Reverse transcriptase PCR indicated that the xcbABC genes form an operon and are cotranscribed divergently from ctrA. XcbA exhibited 52% amino acid similarity to SacB, the putative capsule polymerase of meningococcus serogroup A, suggesting that it plays a role as the serogroup X capsule polymerase. An IS1016 element was found within the intergenic region separating ctrA and xcbA in multiple strains, and this element did not interfere with capsule expression.