Chlorogenic acid (CGA) and its isomer, neochlorogenic acid (NCGA), were found to be the major phenolic compounds in the flesh and peel of three peach cultivars. Their concentrations are especially high in immature fruits (CGA, 151-548 mg/kg; NCGA, 85-380 mg/kg), whose resistance to the brown rot fungus, Monilinia laxa , is very high. The concentrations of these two phenolic compounds decline in maturing fruits (CGA, 77-181 mg/kg; NCGA, 30-82 mg/kg), and this decline is associated with a concomitant increase in susceptibility to brown rot infection. Other phenolic compounds found in the same HPLC chromatograms at 340 nm from each peach extract at varying sampling dates in each of the three peach cultivars were not correlated with the incidence of brown rot and appeared only in some cultivars. The incidence of brown rot for each cultivar at each sampling date was significantly negatively correlated with the NCGA (r > -0.85) and CGA (r > -0.90) contents. At concentrations that are similar to those in peach fruit, CGA does not inhibit spore germination or mycelial growth of M. laxa in culture but markedly inhibits the production of melanin-like pigments in the mycelia of M. laxa in culture (42% melanin reduction). Accordingly, we propose that the high concentrations of CGA and NGA in immature fruits might contribute to their reduced susceptibility or increased resistance to brown rot infection by interfering with fungal melanin production.