Resistance to Xylella fastidiosa was evaluated within a population of 20 interspecific hybrids of Pera sweet orange and Murcott tangor under greenhouse conditions. Efficiency of inoculation, multiplication of bacteria within the plants, xylem vessel morphology, and symptom expression were analysed. The rate of infection ranged from 40 to 100% (average 70%) for all genotypes analysed. Xylella fastidiosa populations ranged from log 0 center dot 59 to log 2 center dot 13 cells mg(-1) tissue for the resistant hybrids. These values were significantly different (P = 0 center dot 05) from those obtained for the tolerant (no symptoms but bacteria recovered) or susceptible (symptoms and bacteria recovered) hybrids (log 3 center dot 02 to log 4 center dot 06 cells mg(-1)). Xylella fastidiosa was recovered from all hybrids (log 2 center dot 31 to 5 center dot 03 CFU mg(-1) tissue) except the resistant ones. The first foliar symptoms appeared at least 90 days post-inoculation, the time varying according to genotype. No correlation between xylem vessel morphology and disease expression was observed, indicating that the resistance was the result of a genetic response of the host. According to this hypothesis, a high broad-based heritability index for resistance was obtained (0 center dot 96) at 210 days from X. fastidiosa inoculations, using bacterial quantification by real-time PCR, which indicated that the influence of the number of bacteria was the result of genetic rather than environmental variations.