Five seagrass species [Halophila ovalis (R.Br) Hook. f., Halodule uninervis (Forsk.) Aschers., Zostera capricorni Aschers., C ymodocea serrulata (R.Br) Aschers. (ed.) and Syringodium isoetifolium (Aschers.) Dandy] from Moreton Bay, Australia, were grown under increased (+25%) and ambient levels of ultraviolet (UV) radiation and photosynthetically active radiation (PAR), and various morphological and physiological responses were examined. Leaf fluorescence ratio (variable:maximum fluorescence) in conjunction with xanthophyll pigment content (violaxanthin, antheraxanthin and zeaxanthin) were used as a measure of photosynthetic efficiency. In addition, absorbance in the UV spectrum, chlorophyll content and chloroplast density were used as indicators of photosynthetic capacity. The seagrass species examined had varying degrees of sensitivity to UV radiation. Halophila ovalis and Halodule uninervis were the most sensitive species, exhibiting the largest decrease in photosynthetic efficiency and chloroplast density and the smallest increase in UV-blocking pigments in response to UV radiation. The more UV-tolerant species, Z. capricorni, C. serruIata and S. isoetifolium, were only significantly affected by increased levels of UV radiation, showing a gradual decline in photosynthetic efficiency and chloroplast density and the largest increases in UV-blocking pigment. UV sensitivity corresponded with leaf morphology, with thicker leaves (as in Z. capricorni, C. serrulata and S. isoetifolium) providing greater morphological protection for UV-sensitive organelles. Not all species were significantly affected by increasing PAR, with decreases in fluorescence ratio and increases in zeaxanthin content observed only in Communicated by G. F. Humphrey, Sydney