The CO2 assimilation rate the transpiration rate (E), the stomatal conductance (gg), C index and the instantaneous water use efficiency of attached leaves háve been estimated in sixteen cultivars of subterranean clover (Trifolium subterraneum L.) fíeld growing plants, both irrigated and subjected to water stress, during latě spring in Mallorca, Spain. Variability among the genotypes with regard to the and other related parameters was highly significant for tiie irrigated plants. Drou^t promoted a marked decrease in both gg and ®ven when the relative water content of leaves was only slightly decreased. For water-stressed plants, genotypic variability in E and C index was also statistically significant. However, variability among the genotypes with regard to gg and was no longer evident under drought. The C index might represent a valuable parameter for evaluation of the genotypic responses of plants to drought.