Pseudomonas syringae, but not P. morsprunorum was detected as an epiphyte on two cherry orchards in Curicó and in other near Santiago, Chile. The identification of both species, was essentially made on the basis of the GATTa test. Most of the isolates recovered from plant surfaces were GATTa+, and they infected inmature sweet cherry fruits and induced a hypersensitive reaction on tobacco. Significant differences (P < 0.01) were found in the size of the lesion produced by 15 GATTa+ isolates tested at 107, 105, 103 and 101 c.f.u./ml inoculum concentration. As the inoculum was diluted, the size of the lesion decreased. Some isolates did not infect fruits at 105 c.f.u./ml, in contrast with other isolates which were still pathogenic at 101 c.f.u./ml. These results suggest the presence of different bitypes within the epiphytic population of P. syringae. These biotypes are morphologically undistinguishable, but the are different in virulence and in their pathogenic behavoir at different inoculum concentrations.