Watermelons (Citrullus vulgaris Schrad.) are in general provided with numerous seeds: large in the "American" varieties and small in the "Japanese" varieties. Triploids (2n=33) of the "Japanese" varieties are already under cultivation by American farmers who import the hybrid seeds from Japan and seedless fruits are being produced in ever increasing amounts. In Brazil the Japanese diploid (2n=22) varieties are only of recent introduction; among the American varieties (2n=22), "Keckley Sweet" is one of the best known and preferred for its good taste. For the obtention of seedless watermelons it seemed advisable to start the work with such a variety because of its commercial significance. The first step was the obtention of tetraploids (2n=44); the adopted method consisted in the treatment of germinating seeds in an aqueous colchicine solution; best results were achieved using an 0.1% solution for 8 to 12 hours. The tetraploids are of more stunted growth, with larger leaves and flowers; the fruits are smaller, round and few-seeded. In attempting to cross tetraploids with diploids, successfull results were obtained when ♀ 4n flowers were pollinated with pollen from 2n plants. Triploid (2n=33) plants, obtained from hybrid seeds, were completely male sterile but set fruits when pollinated with pollen from 2n plants. The triploid plants are somewhat intermediate between di- and tetraploids as to the size of leaves and growth habit; the fruits are less elongated than in the diploids and not round as in the tetraploids. Some fruits are seedless, i.e. with aborted seeds that can be eaten together with the pulp; others show from few to many empty seeds which are flacid, but not quite fit to be eaten. Comparative studies have been made of pollen, stomata and seeds of 2n, 3n and 4n plants. Studies are in progress to solve some problems encountered in the mass production and germination of tetraploid and triploid seed; up to now they are handicaps to the commercial utilization of these "seedless watermelons".