This study investigates lightning density in relation to burning frequency in five South African biomes. Data from automatic lightning flash counters distributed throughout South Africa are used to estimate the range and mean values of annual lightning ground-strike density in each biome. The lowest average lightning frequencies were recorded in the karoo and fynbos biomes, while the highest values were obtained in the 'sour' and inland mountain divisions of the grassland biome, with intermediate values computed for the savanna-woodland and forest biomes. These results were compared with published findings on the effects of different burning frequencies on plant communities in each biome. In general, plant communities occurring in areas experiencing the highest annual lightning densities (e.g., 'sour' and 'mixed' grassland, and moist savanna-woodland) tolerate and require more frequent burning than those typical of areas subject to lower lightning densities (e.g., fynbos, forest, 'sweet' grassland, and arid savanna-woodland). These findings suggest that the constituent plant populations in each biome have adapted to burning frequency according to the local probability of ignition by lightning in the areas they inhabit, and that present day responses of plant communities to burning reflect their ancestral exposure to fire in the course of their evolutionary development under pre-existing, 'natural' fire regimes.