Net photosynthesis was measured under field conditions in Maryland, U.S.A. on several days in the C4 species Amaranthus hybridus and Portulaca oleracea, and the C3 species Chenopodium album. Two similar fully exposed canopy leavesof the same specieswere measured each day, following ambient patterns of air temperature and irradiance. One leaf of the pair was exposed to air at the ambient humidity, while the other leaf was exposed to wetter air, such that the leaf to air water vapor pressure difference was 0.5 to 0.6 of that of the leaf at ambient humidity. Daily totals of net photosynthesis were consistently higher for leaves at increased humidity in C. album, with a mean ratio of 1.17. In contrast, daily totals of net photosynthesis in the two C4 species were at most 1.06 times higher for leaves at increased humidity, with a mean ratio of 1.03. Leaf conductance to water vapor increased as water vapor pressure differences decreased in C. album and A. hybridus. In A. hybridus increased sub-stomatal CO2 concentrations resulting from increased humidity only slightly increased net photosynthesis.