The carbon isotope discrimination (delta C-13) of leaves has been shown to be correlated with the transpiration efficiency of leaves in a wide range of species. This has led to delta C-13 being used in breeding programs to select for improved transpiration efficiency. The correlation between 813 C and transpiration efficiency was determined under well-watered conditions during the vegetative phase in six genotypes of lentil (Lens culinaris Medikus), six genotypes of chickpea (Cicer arietinum L.) and 10 cultivars of narrow-leafed lupin (Lupinus angustifolius L.). Biomass (dry matter) accumulation and water use (transpiration) varied among the genotypes in all three species and transpiration efficiency was 40% to 75% higher in the most efficient compared with the least efficient genotypes. However, delta C-13 and transpiration efficiency were not significantly correlated in any of the species. This suggests that the delta C-13 technique cannot be used in selection for transpiration efficiency in the three grain legumes (pulses) studied.