A previously reported method for polymerase chain reaction (PCR) detection ofGaeumannomyces graminis was modified to simplify it, improve its specificity and decrease the possibility of contamination of the assay. The modified method also allowed discrimination between theG. graminis varieties (tritici andavenae) that are pathogenic to wheat and oats respectively and the variety weakly pathogenic to wheat (var.graminis). An internal positive control for the PCR was also added to the test by including a second pair of primers in the reaction. This positive control has wider application in PCR tests for detection of other fungi.