Multilocus enzyme electrophoresis has shown that a collection of 101 arginine-, hypoxanthine-, uracil-requiring (AHU-) isolates of Neisseria gonorrhoeae, recovered over a 39 year period from the UK and Denmark, were of a single electrophoretic type (91% of strains), or differed from the predominant electrophoretic type at only a single locus. The striking uniformity of the AHU-isolates, and the correlation between auxotype, serovar and overall genetic background, contrasts with previous studies of gonococcal populations (that included very few AHU-strains), and a small sample of non-AHU-isolates studied here, which demonstrated a non-clonal population structure and a lack of association between auxotype, serovar and genetic background. There was no marked difference in the ability of AHU-isolates to be transformed with their own DNA, or with DNA from gonococci of other auxotypes, and the relative genetic stability of AHU-isolates does not appear to be due to a defect in their ability to be transformed. An alternative possibility is that AHU-gonococci recombine with other lineages, but that the resulting recombinants are not maintained in the population. This would occur, for example, if AHU-gonococci competed poorly in mixed infections, within which effective recombination between lineages occurs, and are usually only transmitted from individuals who are singly infected with an AHU-strain. The association between AHU-gonococci and asymptomatic infections may lead to an increased rate of transmission of these strains which under this scenario would be needed to prevent them from being lost from the population.