The insular distribution of distinctive substrates, such as outcrops of serpentine rock, may either promote plant diversity by enhancing opportunities for speciation or reduce diversity by increasing rates of extinction. To examine the relationship between diversity and the spatial structure of habitats, we studied large-scale patterns of diversity in the flora of serpentine in California. We used multiple linear regressions on geographical information system (GIS)-derived data for 85 subregions of the state to analyse the climatic and spatial correlates of plant species richness. The diversity of plants endemic to serpentine declined from north to south and from the coast inland, in association with decreasing rainfall; the same trends were seen in the total flora, but the trends were stronger in serpentine endemics. Diversity of serpentine endemics increased with the area of serpentine and decreased with the mean isolation of serpentine patches in a subregion. The diversity of endemics was not correlated with the number of serpentine patches or their mean perimeter-to-area ratio. We conclude that patchiness in this terrestrial habitat does not appear to promote diversity, even at the large spatial scale associated with speciation and endemism.