This study was conducted to investigate the influence of night temperature on the restoration of fertility in a season-dependent male-sterile tomato mutant (T-4). Plants were grown in greenhouses, in which minimum and maximum temperatures were set at 10°C and 28°C by heating and ventilation, respectively. Flowers were handpollinated and the fruit-set, seed-set, and number of seeds were examined. The rate of fruit-set was high and did not differ much from October to February; almost all fruits formed in October had self-fertile seeds, but 80% of the fruits from November to February were parthenocarpic. The rate of fruit-set dropped from 70% in March to below 10% in May. During this period, most of the fruits were seeded, though fruit-set was low. The number of seeds per seeded fruit varied with the season, being as high as 50 seeds in October, 1–2 seeds per fruit between November and March, and 1–20 seeds per fruit between April and June. A low night temperature of 12°C did not affect fruit-set but resulted in a better seed-set than a high night temperature of 18°C in the greenhouse. Further, pollination of the plants in phytochambers also resulted in a better fruit- and seed-set at 12°C than 24°C. In all cases, the influence of low temperature was more pronounced in autumn than in spring. Fruit-set was 70% at 12°C and 46% at 24°C. Of these fruits, 50% at 12°C and 10% at 24°C were seeded. It was inferred that partial fertility restoration in T-4 can be achieved by manipulation of night temperatures. The female organ was shown to be normal, functional, and compatible with wild-type pollen. From these results, the potential of the malesterile T-4 mutant for use in a two line hybrid-seed production system was apparent.