Size distributions of supraspecific taxa, e.g. genera, measured as the number of included subtaxa, e.g. species, are found to follow a power law. This behaviour has been verified for a number of taxa of different size and taxonomic rank, thus suggesting a fractal structure of biological classifications. This is regarded as probably dependent on evolutionary processes shaping the phylogenetic tree, especially speciation and extinction, as well as on the topological properties of developmental constraints and/or of the ecospace(s) with which the group has been confronted during its history. The role of taxonomic bias is deemphasized.