Eight flax cultivars were evaluated for powdery mildew (PM) resistance under field conditions in 2001/2002 and 2002/2003 growing seasons. The tested cultivars could be divided into two distinct groups. The first group included the resistant cultivars Dakota, Wilden and Williston Brown, while the second one included the susceptible cultivars Cortland, Linore, C.1. 2008, Giza 7, and Giza 8. The difference in disease severity was highly significant (p < 0.01) between any cultivar belonging to the first group and any cultivar belonging to the second group. The tested cultivars were analyzed with a random decamer primer using the polymerase chain reaction (PCR). The primer succeeded in amplifying a total of 20 DNA fragments for the 8 cultivars that were analyzed. Data for PM ratings and amounts of DNA fragments were entered into a computerized stepwise multiple regression analysis. Using the predictors supplied by stepwise regression, a two-factor model constructed to predict PM severity. This model showed that PM severity differences were due largely to the DNA fragments nos. 11 and 13, (699 and 872 bp) which accounted for 97.040/0 of the variation in severity ratings. These results indicate that RAPD analysis may provide a supplementary assay to field tests to distinguish quantitatively between PM resistant or susceptible genotypes.