the initial purpose of this work was to select a mild strain of papaya ringspot virus - type w (prsv-w) from blisters on watermelon (citrullus lanatus) infected with severe strains of the virus. the infectivity of this mild strain, prsv-w-3, on watermelon plants was compared with that of mild strains prsv-w-1 and prsv-w-2, which were previously obtained from blisters formed on mosaic leaves of zucchini squash (cucurbita pepo) cv. caserta. the infectivity of these mild strains was also evaluated based on the concentration of inoculum, the species source of the inoculum and the age of the watermelon plant inoculated. a severe strain isolated from zucchini squash (prsv-w-c) was used as control. the effect of the concentration and the species source of inoculum of the mild strains on the infectivity of watermelon plants were studied with inocula extracted from four, eight, 12 and 16 leaf discs of zucchini squash and watermelon plants, separately infected with the mild and severe strains, diluted in 2 ml of phosphate buffer. three stages of development of watermelon plants were tested for the infectivity with the mild strains. inoculum was prepared with extracts of 12 leaf discs diluted in 2 ml of buffer. the rate of infection of mild strain prsv-w-3 was similar to that of mild strains prsv-w-1 and prsv-w-2 in all infectivity tests, irregardless of the concentration of inoculum, species source of inoculum or stage of development of the test-plant, which reached 100% in some cases. the infectivity of the mild strains on watermelon was more directly related to the intensity of the abrasion produced by mechanical inoculation than with the above studied variables.