Tomato plants (Lycopersicon esculentum Mill. cv. Carmello) seeded on 3 Dec. 1984 and 17 Jan. and 8 Mar. 1985 were grown under natural or supplementary lighting (high-pressure sodium) of 100 μ·mol·s−1·m−2 (photosynthetically active radiation) from pricking out to transplanting. Plants of the first, second, and third seeding dates grown under supplementary lighting had at transplanting dry weights 6.6, 3.5, and 2.5 times higher, respectively, than plants grown under natural light. The number of leaves formed below the first inflorescences was reduced significantly with supplementary lighting, which also reduced the incidence of flower abortion. Supplementary lighting increased early marketable yields for the 3 Dec. seeding by 100% (from 0.77 to 1.55 kg/plant) and total yields by 10% (from 3.55 to 3.91 kg/plant). No significant differences between lighting treatments could be observed in early and total yields of plants from the last seeding date.