Naturally occurring strains of Xylella fastidiosa Wells et al. that were weakly virulent or avirulent to grapevine were inoculated into the lower internodes of Vitis vinifera L. cv. Carignane plants in the greenhouse and challenge inoculated with pathogenic strains 2 weeks later. Several strains provided some reduction in symptom development. Most effective in preventing or slowing the development of Pierce's disease in the greenhouse tests were a strain from sycamore and two from elderberry. In a 2-year test on 'Himrod' grape in the vineyard, strain Syc86-1 (from sycamore), but not strain PD-1 (from grapevine), was effective in limiting the development of Pierce's disease. In tests on new vineyard plantings of 'Flame Seedless' and 'Cabernet Sauvignon', six strains of X. fastidiosa were evaluated for biological control of the natural progression of Pierce's disease. Only strain EB92-1 (from elderberry) provided good control of the disease in both Flame Seedless and Cabernet Sauvignon. Strain Syc86-1 was ineffective in these vineyard tests. Grape strain PD95-6 did lower disease severity in Flame Seedless when compared with nontreated vines, and grape strain PD91-2 delayed symptoms in Cabernet Sauvignon for 12 to 18 months. Biological control by inoculation of susceptible grapevines with benign strains of X. fastidiosa, especially strain EB92-1, appears to have the potential to control Pierce's disease in commercial vineyards in Florida and other areas where the disease occurs.