Two profiles of mitochondrial DNA (mtDNA) were detected among 59 isolates of Pseudocercosporella herpotrichoides from diverse geographical sources. One profile (type II) was present only among isolates pathogenic on rye (R-type), and a second profile (type I) was present in W, C and S-pathotypes of the fungus. Mitochondrial DNA from P. herpotrichoides hybridized strongly to DNA of P. anguioides, but very weakly to DNA from P. aestiva, suggesting that the latter is not closely related to P. herpotrichoides. Fifteen profiles of ribosomal DNA (rDNA) were observed among isolates with type I mtDNA, and five of these were only found among seven isolates from Ireland. The other profiles present among type-I isolates appeared to be largely independent of geographic origin. Three rDNA profiles were observed among R-type (type II mtDNA) isolates.