Kinzie (1993) demonstrated reductions in areal chlorophyll concentrations in Montipora verrucosa as a result of elevated UVR (280—380 nm) while Lesser (1989) observed reduced chlorophyll fluorescence in anemones exposed to elevated UVR (280—400 nm). Recent experiments (Banaszak and Trench 1995) using two species of zooxanthellae (Symbiodinium californium and Symbiodinium microadriaticum) in culture showed that synthesis of chlorophylls a and c2 was significantly depressed (compared to PAR treatments only) after 28 days UVR exposure in S. microadriaticum but not in S. californium. In other experiments bleaching-type responses in tropical anemones and zooxanthids have been evoked by exposure of animals to elevated PAR (400—700 nm) only (Lesser 1989; Lesser and Shick 1989; Lesser et al. 1990). Bleaching has also been observed in the coral Stylophora pistillata by sudden exposure of colonies, previously grown at 25% radiance, to full sunlight (Hoegh-Guldberg and Smith 1989). In other studies (Brown et al. 1994; Fitt andWarner1995) naturallevelsof UVBwerenot thought to be a major factor eliciting bleaching with authors attributing other wavelengths such as PAR and UVA and blue light respectively as proximal causes.