In two citrus groves in southern Florida where blight is severe, mature trees with blight symptoms were sampled and tested for the presence of Xylella fastidiosa by means of enzyme-linked immunosorbent assay (ELISA). Root and stem extracts were obtained monthly from 12 randomly selected trees with blight from each grove. In both groves, there were two yearly peak periods of detection of X. fastidiosa, midsummer (June-August) and midwinter (December-February). Bacteria were not detected in tree extracts in April, May, or October. The seasonal pattern and frequency of detection were similar for both root and stem extracts. Strains of X. fastidiosa could be cultured from fewer than 10% of the samples that were positive by ELISA. The frequency of detection of X. fastidiosa by ELISA varied with the scion/rootstock combinations, with detection most frequent in Pineapple sweet orange on rougn lemon rootstock and least frequent in Pineapple on Cleopatra mandarin.