A study was made on the Pythium isolated from diseased young cucumber plants in Hukuoka Prefecture and from soft rotten fruits of cucumber which was collected at Taikyu, Tyosen. The fungus in question appears to be undescribed. It, therefore, is described new with the following diagnosis: Pythium cucurbitacearum n. sp. Hyphae on culture media variable in width, mostly large, frequently deformed, 6-10μ in diameter, with granular content: aerial hyphae void of granular content, slender, filamentous, 4μ in diameter: zoosporangia mostly spherical, or egg-shaped provided with papilla, measuring 20-32×20-30μ (developed on host plant), and 12-15×10-13μ (on corn agar); oogonia spherical, rich in protoplasma, 22-25μ in diameter; antheridia basal, nearly spherical: the minimum temperature for growth being slightly below 10°C, optimum temperature about 33°C, and maximum temperature slightly below 40°C; causing damping-off of seedlings of cucumber (Cucumis sativa L.), and oriental pickling melon (Cucumis melo L. var. conomon Makino), and soft rot of fruits of watermelon (Citrullus vulgaris MAX.), melon (Cucumis melo L.), and rag gourd (Luffa aegyptica MILL.); occurring in Hukuoka Prefecture and Tyosen.