Isolates of Phytophthora infestans (Mont.) de Bary were collected from potato fields located in main cultivation regions in Egypt during two successive seasons, 2006 and 2007. The ITS4 and ITS5 primers were used to study the genetic variation among the five isolates. Resulted pattern cleared out the differences among isolates concerning the density of the bands and the number in bands for each isolate. A molecular approach was used in order to identify the mating types. Genomic DNA of P. infestans obtained from mycelium grown in pure culture was amplified with the specific primers W16-1/2 and subsequently digested with HaeIII restriction enzyme. The results revealed the presence of the A2 mating type in 1 isolate (Salhia, Behira) among 5 isolates obtained from commercial fields in Egypt. Also, since it is observed that fungicides containing metalaxyl are loosing their efficacy in many fields, when in vitro tests for metalaxyl resistance in P. infestans isolates were performed. Results revealed strong presence of the metalaxyl tolerant and resistant phenotypes in Egyptian P. infestans population. Importance of the genetic studies to follow pathogenicity and population structure of P. infestans was noticed.