Temperature, cultivar, locus of inoculation, and prior-inoculation (inoculation 24 hr earlier with 3 nonpathogenic fungi) affected the expression of fusarium wilt caused by Fusarium oxysporium f. niveum (E.F.Sm.) Snyder & Hansen, in watermelons. Prior inoculation with Helminthosporium carbonum Ulktrup was found to be more effective in cross protection than were Fusarium oxysporium f. lycopersici (Sacc.) Snyder & Hansen, or Verticillium alboatrum Reinke & Berth. Temperatures of 27°C resulted in more fusarium infection than at 20° and obscured the expression both of genetic resistance and resistance induced by the prior-inoculations. Induced resistance increased plant survival and plant growth for 3 cultivars in field trials, but possibly produced deleterious effects in a resistant cultivar.