Pythium leak is a post-harvest disease of potato tubers manifested by a wet, spongy rot that can quickly lead to complete tuber decay.Two isolates of Pythium sp. from Salhia and Ismailia districts were isolated from naturally infected potato tubers. The morphological characters examined were dimensions of oogonia ,oospores , antheridia , hyphal swelling and oospores wall thickness. Oogonia were mostly terminal and the hyphal swelling were mostly intercalary in the two isolates. These morphological characters of Salhia and Ismailia isolates were in accordance with the reference data of Pythium ultimum var, ultimum. . Pathogencity tests of the two P.ultimum isolates of Salhia and Ismailia revealed that Ismailia isolate capable to cause higher incidence of infected tubers and the depth of internal rot was more than that in case of  Salhia isolate. Cultivar reactions indicated that Diamant cultivar was the most susceptible to both P.ultimum isolates, however, Maradona and Baraka cultivars were the least susceptible ones. In the same time, Spunta , King Edward, Alpha ,Desiree and Drage showed moderately reactions . The optimum temperature for growth  of both isolates of  P.ultimumgrowth on PDA was 30 ^oC . The lowest  growth was observed at 5 and 40 ^oC.  Laboratory screening tests with four fungicides indicated that Previcure-N inhibited completely at 150 ppm.Accorrding to the   available literature knowledge; this is the first report of infection with leak rot of potato tubers caused by P. ultimum var. ultimum  in Egypt.