Rhizoctonia solani AG-3 causes considerable yield loss in potato fields in eastern Canada and U.S.A. The accurate identification of AG-3 isolates is strategic prior to planting potatoes. To obtain a fast and reliable identification method, RAPD experiments were carried out to obtain specific genetic markers of AG-3 isolates. DNA from various isolates of R. solani was submitted to RAPD amplification using random 10 mers primers. One of the forty primers used yielded a 2·6 kbp fragment present in all isolates of AG-3. The specificity of this fragment was assessed by Southern blot analysis. It was partly sequenced and DNA primers were designed for PCR amplification experiments. A PCR-based restriction mapping method, using one restriction endonuclease, Xho I, was developed for specific detection of AG-3 isolates. Analysis of data showed that AG-3 isolates were distinct to other AGs R. solani. The detection method described here is very specific and reliable; it can be applied on plant tissue and soil infected with R. solani AG-3.