Combining ability among tetraploid and diploid lines of watermelonCombining ability of three tetraploid (LT7-48.1, LT9-24.1 and LTCC-24) and four diploid lines of watermelon ('Crimson Sweet', 'Pérola', 'New Hampshire Midget' and 'Charleston Gray') was evaluated using a partial diallel scheme. A randomized block design, with three replications and seven plants in each field plot was used. Parents and F 1 's were evaluated regarding the number of days to the set of the first male and female flower, the number of fruits per plant and the length of the main vine, fruit mean weight, total soluble solids, transversal and axial fruit diameter, rind thickness at the stem and at the scar flower regions, flesh color and hollow heart. The tetraploid and diploid parents were diversified regarding the traits studied. The GCA effects were much higher than the SCA effects for most of the characters analyzed, particularly, for characters related to plant yield, showing the major relevance of the additive genetic effects. However, some traits like earliness and hollow heart presented non-additive genetic effects and, therefore, evaluation and selection of lines in hybrid combinations will allow the synthesis of superior watermelon triploid hybrids.