Alternaria rot caused by Alternaria alternata and A. arborescens is a major postharvest disease of blueberries in California. Natamycin, a food additive and biochemical fungicide, was evaluated for its potential as a postharvest treatment to control Alternaria rot. The baseline sensitivities of A. alternata and A. arborescens were established. The effective concentrations of natamycin for 50% inhibition of conidia germination relative to control (EC₅₀) of 34 A. alternata isolates ranged from 0.640 to 1.032 µg/ml with a mean of 0.778 ± 0.097 µg/ml and from 2.660 to 4.330 µg/ml with a mean of 3.390 ± 0.435 µg/ml for reduction of mycelial growth. EC₅₀ values of natamycin for 34 A. arborescens isolates ranged from 0.540 to 1.335 µg/ml with a mean of 0.950 ± 0.162 µg/ml for inhibition of conidial germination, and from 0.610 to 3.430 µg/ml with a mean of 1.920 ± 0.637 µg/ml for mycelial growth. Control tests on detached blueberry fruit inoculated with four Alternaria isolates showed that even a low concentration of natamycin (230 µg/ml) significantly reduced disease incidence and severity of Alternaria rot, regardless of blueberry variety, application method or incubation condition. Our results suggest that natamycin can be an effective postharvest treatment for control of postharvest Alternaria rot in blueberry.