SummaryThe semi-dominantly acting allele of the Mlg locus in barley confers a race-specific resistance to the causal agent of powdery mildew, Erysiphe graminis f.sp. hordei. High resolution genetic mapping via RFLP analysis enabled us to identify a set of markers on chromosome 4 which are either tightly linked or cosegregate with the resistance function. Comparison of marker distances in segregating F2 populations originating from different genotypes revealed an unexpected variation of recombination frequencies in the vicinity of the locus. Based on near-isogenic lines carrying either resistance gene Mlg or Mia12 and a cell-type specific analysis it was demonstrated that Mlgacts at an earlier stage of fungal development than MIa12. A time-course analysis of the early infection process combined with gene dosage experiments provided strong evidence that attacked cells retain viability after the defence response and that hypersensitive cell death (HR) is a secondary consequence, but not causally required for Mlg-mediated arrest of fungal growth. It is speculated that gene dosage experiments may provide a means to separate primary from secondary defence reactions.