The population structure of Neisseria meningitidis was investigated by MLEE and REA. Using MLEE 107 strains were characterised studying 6 loci. Clones were defined as 5 loci were identical in the strains studied. Seventy two strains fell into twenty six multimember clones. The clones identified contained between 2 and 6 members. The remaining (n=35) strains were regarded as unique. The genetic diversity of the population was estimated as 0.700. This high degree of diversity seems to be because the majority of the strains were isolated from sporadic cases. Isolates within multimember clones contain isolates of outbreaks and sporadic cases. Clones were also identified in a collection of serogroup C. Neisseria meningtidis (n=34) strains by REA using endonuclease StuI. Eleven multimember clones were recognised containing between 2 and 15 members. Seven strains were regarded as unique. The largest multimember clone (n=15) contained 6 rifampicin resistant meningococci and also strains sensitive to rifampicin. This finding seems to be in the favour of this hypothesis that the resistant phenotype arose once and spread through the United Kingdom. Similarly matrices between 11 serogroup C and 5 serogroup B meningococcal strains were determined. The extensive Dice similarity coefficient between strains of serogroup C and B and also close genetic distance between these two serogroups, on the one hand, and very low Dice similarity coefficient (>50%) and distant genetic relations between serogroups C meningococcal isolates, on the other, demonstrated serogrouping cannot be regarded as a reflection of overall genetic similarity; although its practicality in epidemics and its convenience renders it useful as a first step in hierarchical typing system.