A method is described which allows the diversity of agarics in a site to be determined as an index, the Site Diversity Index (SDI). For this the individual species recorded from a site are given a score which is that of the frequency of recording class to which it has been assigned, that score increasing with increased rarity of recording in the BMS database. The methods of defining the classes of frequency of recording, the score accorded to a class, and the calculation of the SDI are described. Examples of the SDI are given for several sites in the Forest of Dean as well as for other sites and areas. Site Profiles illustrate the extent to which each of the classes has been completed. The flattening of the rate of accumulation of species is discussed. Further progress will involve the widening of the range of included species to those of other classes of basidiomycetes. The method can be used to rank sites and, in the early stages of a survey and given the meeting of two criteria, to prediction of future diversity of a site. Further use in respect of Important Fungus Areas and conservation sites is envisaged.