studies were carried out to evaluate the protective effect of two mild strains of papaya ringspot virus, watermelon strain, on watermelon (citrullus lanatus) plants under greenhouse and field conditions. the results showed that protection was positive under both conditions. however, protected plants of 'crimson sweet' showed a reduction on the average weight of fruit per plant of, approximately, 50% and 10.8%, as compared to the yield of healthy plants, in the first and second trials, respectively. yield reduction on protected 'crimson tide' was approximately 36.4% in one trial. in addition to yield reduction, another problem associated with cross protection in watermelon was the requirement of two successive inoculations of the seedling, on consecutive days, to increase the frequency of protected plants.