Biogeographical regions of Switzerland were defined using numerical analyses based on detailed information about the distribution patterns of the plant species occurring in the area. The reference database includes 1876 species and 350 mapping areas. When comparing the resulting floristic regions with the standard biogeographical regions according to Gutersohn (1973), two major differences were found, both with regard to the delimitation of the Central Alps. For questions about flora or vegetation, the floristically defined regions seem to be more appropriate. In addition, the reference database was reduced according to various criteria in order to find out how many species are needed for a satisfactory regionalization. As expected, results were best when high numbers of species were used; when selected systematically, at least one third of the species present are required (623 species). Very good results were also obtained with as few as 1-6% of the flora (twenty to 160 species), so long as the species were selected either according to their statistically established discriminating power or according to expert knowledge. With the expert approach, moreover, only easily identifiable species were used. This indicates that floristic regions can be successfully delimited even if detailed distribution maps are not available and even if non-experts participate in the mapping. Data-sets consisting of species selected according to taxonomic or ecological criteria turned out to be less suitable for general regionalizations. However, they may be useful for addressing specific ecological questions.