Pythium ultimum was readily isolated from the necrotic roots of young and mature muskmelon plants growing in soil in which root rot had been severe. Muskmelons planted in steamed soils inoculated with P. ultimum developed necrotic roots and aboveground symptoms closely similar to those produced in naturally infested field soil. The fungus was considerably more pathogenic at low than at high soil temperatures. All of seven commercial varieties of muskmelon commonly grown in southern Ontario were moderately to highly susceptible to P. ultimum. Experimental data support the conclusion that P. ultimum probably plays an important role in the "sudden wilt" disease of mature muskmelon plants.