Photosynthetic gas exchange, plant-water relations characteristics, and stable carbon isotope discrimination (Delta) were evaluated for five Coffea arabica L. genotypes growing under two soil moisture regimes in the field. The Delta of leaf tissue was strongly correlated (r = -0.95) with inherent water use efficiency (ratio of assimilation to stomatal conductance; A/g). The variation in inherent water use efficiency (WUE) among genotypes was 30% for plants irrigated weekly. The higher WUE exhibited by some of these plants resulted from reduced g rather than increased photosynthetic capacity at a given g. Withholding irrigation for 1 month caused Delta to decline substantially in expanding leaf tissue of all genotypes. A strong correlation (r = 0.92) was found between Delta and plant hydraulic efficiency estimated as the ratio of g to the diurnal range in leaf water potential (Psi(l)). The Delta values for plants irrigated weekly adequately predicted drought-induced changes in Delta (r = 0.99) and midday Psi(l) (r = 0.95). The results indicated that Delta might be used to evaluate several aspects of plant performance and response to specific environmental conditions, once suitable background physiological data have been gathered.