AbstractTheHolcuscomplex in France consists of two species,Holcus lanatusL. (Yorkshire fog; 2n= 2x= 14) andHolcus mollisL. (Creeping soft‐grass; 2n= 4x= 28) and an interspecific hybridH.m.×H.l.(2n= 5x= 35), which is morphologically similar toHolcus mollis.A heterologous rDNA probe from wheat was used to detect the corresponding region in Holcus (s. l.) genomic DNA'fragments, for six to eight plants from 13 populations located south‐west of Paris. A restriction enzyme map of the ribosomal RNA genes (rDNA) inHolcus(s. l.) was also constructed. The length polymorphism detected in the IGS region was used as a DNA fingerprint for the identification of different cytotypes and species of theHolcuscomplex and for the typing and delimitation of individuals in populations. In the light of the results we reconsider the assumption that the pentaploid hybridH.m.×H.l.is purely clonal. New hypotheses concerning the origin of the pentaploid hybrid and its reproduction are proposed, and the consequences for genetic diversity in natural populations disc