Effects of CO2 enrichment on photosynthesis and on dry matter allocation were examined in two tobacco (Nicotiana tabaum L.) genotypes. Samsun and W38. Plants were grown from seed in controlled environment chambers at a photosynthetic photon flux density of 450 μmol m−2 s−1. Averaged over the 9 day study, net photosynthesis rates were 14.2±0.5 and 13.0±0.4 μmol m−2 s−1 in elevated (70 Pa) and in ambient (35 Pal CO2 air. respectively, when measured at the irradiance and CO2 partial pressure employed for plant growth. However. photosynthesis rates of plants grown in elevated CO2 were 50% less than those of the ambient controls on the last day of treatment, when measured al 70 Pa CO2 air and an irradiance of 900 μmol m−2 s−1 Total plant dry weight and specific leaf weight were greater (P < 0.051 in enriched-CO2-grown than in ambient-CO2-grown plants. Leaf starch, measured during the first hour of the photoperiod. increased over 7 days of treatment in elevaled-CO2-grown but not in ambient-CO2-grown plants. Ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) activities of tobacco plants grown at 35 and 70 Pa CO2 air were 58.5±4.5 and 48.5±3.7 umol m−2 s−1. respectively, between days 0 and 9 of the study. Rubisco activation state. Rubisco protein concentration. soluble protein and total chlorophyll were unaffected by CO2 enrichment. The above findings demonstrated that photosynthesis was down regulated in tobacco plants after 7 to 9 days of CO2 enrichment at low photosynthetic photon flxu density, but less than at moderate irradiances.