The effect of water stress was examined in a clone of Acacia auriculiformis grown in irrigated and unirrigated sites during the wet and dry seasons in Darwin, northern Australia. During the dry season unirrigated plants had very low leaf water potential and thick leaf cuticles compared with irrigated plants. The plants were photoinhibited under these conditions and they showed greatly reduced Fv/Fm ratio and photosynthetic activity. Chlorophyll and leaf soluble protein contents decreased but carotenoid and xanthophyll, especially zeaxanthin, contents increased greatly compared with irrigated plants. Unirrigated plants showed large diurnal changes in photosynthesis and zeaxanthin and violaxanthin contents. Photosynthetic activity was 3-7-fold less and zeaxanthin level 20-50-fold higher than the values determined in the wet season or under irrigated conditions. The high level of xanthophyll cycling observed in the unirrigated or dry season plants was presumably associated with the dissipation of excess light energy and serves to protect the photosynthetic apparatus from photoinhibition.