The gas exchange characteristics are reported for Amaranthus tricolor, a C4 vegetable amaranth of southeastern Asia. Maximum photosynthetic capacity was 48.3±1.0 μmol CO2 m(-2) s(-1) and the temperature optimum was 35°C. The calculated intercellular CO2 concentration at this leaf temperature and an incident photon flux (400-700 mm) of 2 mmol m(-2) s(-1) averaged 208±14 μl l(-1), abnormally high for a C4 species. The photosynthetic rate, intercellular CO2 concentration, and leaf conductance all decreased with an increase in water vapor pressure deficit. However, the decrease in leaf conductance which resulted in a decrease in intercellular CO2 concentration accounted for only one fourth of the observed decrease in photosynthetic rate as water vapor pressure deficit was increased. Subsequent measurements indicated that the dependence of net photosynthesis on intercellular CO2 concentration changed with water vapor pressure deficit.