These studies were carried out to identify varieties with a high female flower bearing ability among genetic resources and develop a simple method to evaluate the female flower bearing ability of watermelon before transplanting. The results showed that Japanese F1 varieties bore more female flowers than the other varieties except for those from China and Taiwan. Also varieties with many female flowers among wild species could be used as breeding materials to improve the female bearing ability of other watermelon varieties. Watermelon varieties with a high female flower bearing ability bore the first female flower at a lower node than those with a low female flower bearing ability, when sprayed with silver thiosulfate, STS, at the cotyledonary stage. Correlation between the number of female flowers in the F2 population sprayed with 6 mM STS and the node order of the first female flowers was high. In a progeny selected from the F2 population, the female flower was borne at a lower node order of the main stem, whereas in another, the first female flower differentiated at a higher node order; the former bore more female flowers than the latter. These results indicate that it is possible to select plants with a high female flower bearing ability by STS treatment before transplanting.