Allozyme diversity was surveyed at 16 loci across 16 populations of the monospecific, but morphologically diverse, endemic shrub Geleznowia verrucosa Turcz. (Rutaceae). Single-locus diversity measures portrayed G. verrucosa as a genetically depauperate genus (A, 1.4; P, 29.6%), with total genetic diversity (HT, 0.304) partitioned between populations (DST, 0.175), rather than within (HS, 0.129). Some 58% of the total genetic diversity was attributable to interpopulational differences. Although restricted distribution, bottlenecks and/or founder effects and small population size have probably contributed to the low levels of genetic diversity found within this genus, it is evident from this investigation that the genus is not monospecific. The partitioning of genetic diversity, and possible differences in reproductive strategy, suggest that the small- and large-flowered forms represent distinct taxa. Higher levels of divergence between the intermediate-form populations and increased heterozygosity at the Lap locus suggest that this form is of hybrid origin and has arisen through a series of hybrid events between the small and large forms.