The carbon isotopic composition (δ¹³C) of woodand leaf cellulose of beech trees (Fagus sylvatica L.) was studied at 80 sites in northeastern France. We sampled sites withcontrasting water balance, depending on soil type and precipi-tation. We tested the hypothesis that inter-site variations in plant δ¹³C reflect the spatial distribution of soil water availability, and we assessed whether δ¹³C could be used as a bioindicator of soil water availability. Patterns of variation in δ¹³C were compared with estimates of monthly water balance and with other soil characteristics. Between-site variability in δ¹³C was high (2.9‰ range in wood cellulose, 2.1‰ in leaf cellulose), but variation in water availability appeared to be only aminor factor contributing to this variation in δ¹³C. Unexpectedly, spatial variations in wood and leaf cellulose δ¹³C were significantly and positively related to soil fertility expressed by soil pH (r= 0.42 and 0.43, respectively) and cation content. On average, trees growing on acidic soils displayed 0.5‰ lower δ¹³C in both wood and leaf material than trees growing on neutral or calcareous soils. Our initial hypothesis of a strong negative relationship between δ¹³C and site water availability was not confirmed. In the study zone, neither wood nor leaf δ¹³C appeared to be a reliable bioindicator of spatial variations in water availability. Possible causes for the lack of a relationship arediscussed. Our findings confirm, under natural conditions, thestrong effect of soil fertility on water-use efficiency previously observed in experiments. This effect needs to be considered in isotopic studies involving different sites.