Scarcity of water resources is a major component of low productivity of crop plants in arid and semi-arid climatic conditions. Among all biotic and abiotic stress factors, drought accounts for nearly 45% of the decreases in crop yield. Therefore, a 3-replicate randomized complete block factorial pot experiment was conducted in 2011 at College of Agriculture, Isfahan University of Technology, Isfahan, Iran. The experiment was aimed at studying the effects of water deficit stress on gas exchange properties, chlorophyll concentration (Chl), chlorophyll fluorescence, leaf proline and soluble carbohydrates content, and some morphological characteristics of roots of sesame genotypes. Four levels of irrigation included irrigation when soil water potential of pots was approximately -0.2 MPa (control) =W 1 , -0.5 MPa (low water deficit) =W 2 , -1.0 MPa (intermediate water deficit) =W 3 and -1.4 MPa (severe water deficit) =W 4 and 12 sesame genotypes included Oltan, Non-branching Naz, Branching Naz, Darab, Yekta, Varamin, Ardestan, Isfarayen, Ferdous-II, Sabzevar-I, Kashmar and Khaf. Carbon exchange rate (A) and stomatal conductivity (g s ) were decreased by 43.84% and 39.65%, respectively, under W 4 compared to W 1 . Leaf proline and soluble carbohydrates content were increased with water deficit; they were increased by 129.8% and 35.6%, respectively, under W 4 compared to W 1 . Chl-a, Chl-b and total leaf Chl content (Chl-tot) were decreased by 72.83%, 49.14% and 72.12% respectively, due to severe water deficit, relative to the control. Maximum quantum efficiency of PSII (F v /F m ) was decreased by 17.72% under W 4 compared to W 1 . Plant height and shoot dry weight indicated 21.53% and 52.18% decreases, respectively, with W 4 relative to W 1 . Sesame root characteristics were negatively affected by water deficit; total root length, root volume, root surface area and root dry weight indicated 72.83%, 49.14%, 72.12% and 39% decreases, respectively, when grown under severe water deficit compared to control. Meanwhile, root/shoot ratio increased by 30.59% under severe water deficit in comparison to under control conditions. No significant effects for interaction of genotypes by irrigation was detected, though, Isfarayen proved more appropriate for water deficit conditions at least to some extent due, apparently, to its ability to synthesize more proline and soluble carbohydrates compounds and produce a more extensive root system, compared to the rest of sesame genotypes used in this study. It could be concluded that plant photosynthetic components and rooting behavior of sesame and, therefore, its dry matter production are negatively affected by water deficit at least at the vegetative growth stages. Key words: water deficit, sesame, vegetative growth stage, root, photosynthesis characteristics