Net photosynthesis ( P n), transpiration ( Tr), diffusion resistance to CO 2 and water-use efficiency of azalea ( Rhododendron simsii Planch. sp.) cultivars ‘Hellmut Vogel’ and ‘Reinhold Ambrosius’ were studied as a function of photon flux density, by gas-exchange experiments in an assimilation chamber. Differences were found in max. P n, in light compensation points, in diffusion resistances and water-use efficiences. ‘Hellmut Vogel’ had a higher P n rate and a lower internal ( r i) and stomatal ( r s) resistance. These factors are thought to be responsible for the field growth vigour of ‘Hellmut Vogel’ as compared to ‘Reinhold Ambrosius’.