There is concern that high temperatures resulting from global warming could reduce fruit set of tomato (Solanum lycopersicum). However, fruit set of parthenocarpic tomato genotypes, which often bears seedless fruit, is not reduced when grown under a high temperature. The cause of seedless fruit development was studied with the aim of increasing the seed number in parthenocarpic tomato. Ovule number at anthesis in parthenocarpic and non-parthenocarpic fruit did not differ, but the proportion of undeveloped ovules increased with time after anthesis in parthenocarpic tomato, whereas most ovules in non-parthenocarpic tomato developed normally. Pollen grains germinated on the stigma and extruded pollen tubes in parthenocarpic and non-parthenocarpic tomatoes, but in parthenocarpic tomato, pollen tube elongation was markedly inhibited in the style base. Elongation of pollen tubes on agar containing indoleacetic acid (IAA) was depressed in parthenocarpic and non-parthenocarpic tomato plants. p-Chlorophenoxyisobutyric acid (PCIB), which inhibits auxin action, did not affect the fruit set and fresh weight in either type of tomato, although seed number per fruit in parthenocarpic tomato was significantly increased from 13 ± 2 to 74 ± 6 seeds by PCIB treatment. These results indicated that a high IAA concentration in the ovary of parthenocarpic tomato inhibited pollen tube elongation, and that poor fertilization resulted in failure of ovule development. Moreover, floral organs in parthenocarpic tomato were normally developed as in non-parthenocarpic tomato, and seed development could be induced in parthenocarpic tomato by PCIB treatment.