A detailed experimental assessment has been made of a wide range of morphological, physiological, pathological and cultural characters of 23 different isolates of 'Helminthosporium' of which 19 belong to the genus Drechslera and combined with information from the literature on a further six forms. The various properties of each fungus have been described by 223 unit characters and the similarity coefficients between all possible pairs calculated using an electronic computer. Similarity matrices and dendrograms have been constructed from these data. It has been shown that inclusion of those characters scored negative in both strains increases, while their exclusion decreases, the similarity coefficient. There were only minor differences between the results of additive and non-additive scoring. Reasons are presented which favour a system in which some characters are scored one way and some the other. Forms showing more than 80% similarity are considered as conspecific, while forms with 60 to 80% similarity are congeneric. H. triseptatum, H. dematoideum and H. biformae form a group distinct from Dreschslera, Bipolaris and Stemphylium which are confirmed as separate genera. H. avenae, H. siccans and H. avenaceum share 95% or more characters in common and appear to be conspecific. D. dictyoides f. dictyoides and D. dictyoides f. perenne are confirmed but are not thought worthy of specific rank. H. dematoideum and H. biformae may be the same species while a new species may be justified for a form isolated from Cocksfoot grass (Dactylis golmerata).