Research on Fusarium wilt of sweet potato, a vascular disease caused by Fusarium oxysporum Schlecht f. batatis (Wr.) Snyder & Hansen, was undertaken to determine the susceptibility of various sweet potato lines to Maryland isolates of the pathogen under field and greenhouse conditions. Highly resistant lines would be useful as sources of resistance to Fusarium wilt in sweet potato breeding programs. In 2 years of field and greenhouse trials, 94 different lines of sweet potato were inoculated with a composite spore and mycelial suspension of 5 Maryland isolates of F. oxysporum f. batatis. Results indicated that the following lines were highly resistant to the pathogen: the foreign plant introductions P.I. 153655 ("Tinian"), P.I. 153906, P.I. 153907, and P.I. 251602; the variety Pelican Processor; and the breeding selections B-6842 from the United States Department of Agriculture Plant Industry Station at Beltsville, Maryland, and T-7 from the Georgia Coastal Plain Agricultural Experiment Station at Tifton, Georgia. Greenhouse experiments showed that the host range of Fusarium oxysporum f. batatis should include an additional species of Morning Glory, lpomoea pandurata (L.) G. F. w. Mey. Furthermore, no symptoms of infection were obtained on various crop plants commonly grown in rotation on land used for sweet potato culture. Physiological studies in the laboratory and greenhouse indicated that no significant levels of fungitoxic substances were present in either uninoculated or inoculated ''Tinian" plants. Nor could a fungal metabolite be detected, under the existing experimental conditions, which was toxic to a susceptible variety of sweet potato (Porto Rico) but not to the resistant ''Tinian". A study was made of the basis for resistance of the foreign plant introduction "Tinian" (P. I. 153655). Histological examinations of serial stem sections of the susceptible sweet potato variety Porto Rico and of the resistant foreign plant introduction ''Tinian" were made from plants collected at 3-day intervals following inoculation with spores of the pathogen. It was found that "Tinian" responded to infection by the production of tyloses in advance of the fungus. Twelve days after inoculation, 75 - 88% of the vessels which were 22 - 32 mn above the invasion site at the base of the plant were completely filled with tyloses. This compared to only 0 - 3% in the uninoculated control plants. Furthermore, no mycelia or spores could be detected in tnis region but were present in 25 - 50% of tne vessels within 11 mm of tne invasion site. In the variety Porto Rico the occurrence of tyloses in the inoculated plants was not significantly greater than in the uninoculated controls, except near the invasion site where after 12 days 3 - 6% of the vessels contained small 1 tyloses. The pathogen was not limited, as in ''Tinian", to the immediate invasion site. Tnis suggests that tne production of tyloses in "Tinian may represent an important defense mechanism against Fusarium wilt.