This study was conducted to clarify the genotypic differences in diurnal variations in leaf gas exchange characteristics and the effect of irrigation levels, growth stages and cultivars on leaf water status, gas exchange characteristics and seed yields. Associations of leaf water status with photosynthetic rate in five cultivars of snap bean were also determined. Diurnally, leaf gas exchange was highest at 9:30 h, decreased but remained constant between 11:30 to 14:15 h and then decreased up to 16:15 h. Cultivars Haibushi and Ishigaki-2 displayed higher photosynthetic activity throughout the day. The cultivars having higher seed yield showed higher leaf conductance, transpiration rate and leaf vapor pressure deficit at floral bud initiation stage and higher leaf water potential and relative water content in the irrigated plot at flowering stage. The cultivars with slow development of leaf water deficit displayed higher rates of photosynthesis and vice versa. The results showed that midday hours showed large genotypic differences in leaf gas exchange characteristics with constancy of photosynthetically active radiation. Therefore, screening of snap bean cultivars can be made during midday hours for gas exchange characteristics irrespective of growth stages and irrigation levels. Cultivars maintaining higher leaf water content with increasing water deficit can be selected for higher rates of photosynthesis