In situ gas exchange measurement in leaves of swamp taro, Colocasia esculenta (L.) Schott, grown in swampy land, was studied during a natural diurnal period (0700 hours to 1600 hours) at peak runner production stage (225 DAP). With initial increase in photosynthetic rate up to 1100 hours there was a mid day decline at 1330-1500 hours, followed by further increase at 1530-1600 hours. The photosynthetically active radiation, (PPFD; R2 = 0.61*), air (R2 = 0.81**) and leaf temperature (R2 = 0.80Â·*) showed significant relationship with photosynthesis indicating importance of light intensity and temperature on photosynthesis rate. The optimum PPFD noted was 1,190 fJ-moleÂ·m-2 S-1 at maximum photosynthesis rate of 9.9778 fJ-mole CO2'm-2 S-I. Similarly optimum air and leaf temperature was found to be 34.4Â°C and 34.6Â°c for maximum photosynthesis rate of 11.1887 and 11.2181fJ-mole CO2 m-2 S-1 respectively. The PPFD (photosynthetic photon flux density) appeared critical for increasing leaf temperature directly (R2 = 0.92Â·Â·) rather than its effect on CO2 fixation rate (R2 = 0.61*). The closer positive association of RH (R2 = 0.69**) with stomatal conductance (g) suggested that CO2 diffusion in swamp taro leaf was controlled by humidity and stomatal conductance while fixation of CO2 appeared under control of light, temperature and other non-stomatal factors.