It is proposed to designate the fungus F. oxysporum f. sp. radicis-lycopersici n.f. sp. The pathogen is morphologically indistinguishable from other isolates of F. oxysporum. The distinctive features of the new forma specialis are that it attacks all species of Lycopersicon tested and all cultivars of L. esculentum tested; it causes a brown rot of cortical and vascular tissue of the hypocotyl and roots that extends upward not more than 25 cm in the stem; it can kill young seedlings, but usually causes wilt as a result of stem girdling at 1st fruiting of tomatoes; and it is favored by cool temperatures.