Allozyme frequency data were used to estimate hierarchical levels of genetic diversity within and among the nine subspecies of the morphologically variable shrub Persoonia mollis R. Br. (Proteaceae). This species is a fire-sensitive component of the fire-prone sclerophyllous vegetation of south- eastern Australia. The total gene diversity (H(T)=0.139) within P. mollis was less than half that typically found within angiosperm species. However, P. mollis was typical of angiosperms in the way its gene diversity was distributed, with 78.3 per cent of the total gene diversity found within populations. Of the 21.7 per cent found among populations, 17.9 per cent was attributed to differences among subspecies and only 3.8 per cent attributed to differences among populations within subspecies. Heterozygosity (H(E)) within populations varied 10-fold from 0.023-0.206, with a comparatively low mean over all populations of 0.109. A significant deficit of heterozygotes for the outbreeding P. mollis is consistent with the 'heterozygosity paradox', is caused by a Wahlund effect, and suggests an average genetic neighbourhood size of between 3 and 60 plants. Explanations for these patterns and the weak correlation of these data with morphological variation are discussed. It is suggested that the high frequency and haphazard occurrence of fire plays a major role in affecting the levels and distribution of genetic diversity within P. mollis, and probably within other fire-sensitive plant species also occurring in fire-prone habitats, through relatively frequent localized extinction and subsequent recolonization.