Rhynchosporium secalis (Oudem.) J.J. Davis is the causal agent of the worldwide disease, barley scald. The global distribution and economic importance of this pathogen have motivated several studies focused on its biology and population dynamics (Zhan et al., 2008). This pathogen is a haploid organism whose teleomorph has never been identified (Salamati et al., 2000; Zaffarano et al., 2006). For organisms with an unknown sexual stage, recombination can only be inferred by indirect methods (Milgroom, 1996; Taylor et al., 1999). In the case of R. secalis, a cryptic sexual stage has been proposed based on indirect evidences (McDermott, et al., 1989; Goodwin et al., 1993; McDonald et al., 1999; Salamati et al., 2000; Linde et al., 2003; Zaffarano et al., 2006). Field populations will be sampleed in Europe and Ethiopia, assayed with microsatellite loci. Model based methods will be used to infer the population structure and recent migration patterns.