In Part I of this paper, the principles of numerical taxonomy are discussed and the practical processes outlined, with particular reference to their applicability to the Fungi Imperfecti.In Part II, one of the authors (J.R.P.) discusses several scoring methods, including one introduced by the authors in which a 'primary' character is weighted according to the number of 'secondary' characters used to describe it. The merits of the various methods are considered.In Part III, one of the authors (W.B.K.) discusses the results obtained when 30 strains of Hyphomycetes belonging to the Leptographium complex were compared using a computer, the taxonomic data being scored in six ways. The classification agreeing most closely with the established one was obtained when 'primary' characters were weighted and 'negative matches' were included. Of the methods applied, this is considered in Part II to be the most logically satisfying.