In five California evergreen trees and shrubs co-occurring in this study but most common in habitats of different moisture availability, leaf nitrogen was a major determinant of photosynthetic capacity. Within each species, stomatal conductance was highly correlated with photosynthetic capacity, resulting in little variation in the concentration of CO sub(2) in the intercellular spaces. Among species, intercellular CO sub(2) concentrations varied significantly. Under controlled conditions, the leaves that realized the highest photosynthesis per unit of leaf nitrogen tended to realize the lowest photosynthesis per unit of water transpired. The ratio of photosynthesis to transpiration, an instantaneous measure of intrinsic water-use efficiency, was highest in the species commonly found in the driest habitats and lowest in the species most common in the wettest habitats.