1. A new technique was developed to extract soluble proteins from the intracellular dinoflagellate symbionts (zooxanthellae) of cnidarians, suitable for analysis by denaturing gel electrophoresis (SDS-PAGE).2. Using this technique, a highly abundant 33 kDa protein has been detected from the zooxanthellae of both a temperate sea anemone (Anemonia viridis) and three tropical corals (Goniopora djiboutiensis, G. pandoraensis and G. stokesi).3. Relative abundance of the protein varies with thermal shock, season, habitat from which the specimens were collected and laboratory holding conditions.4. Amino acid sequence analyses and searches of current CD-ROM databases failed to find a significant match for a 31 residue amino-terminus sequence or an 8 residue internal sequence, but showed a partial (9 residue) homology with the vertebrate high-mobility group (HMG) nuclear proteins which are involved with DNA binding and unwinding during replication. We are currently seeking funds to further investigate this apparently novel protein, with a view to assessing its potential role as an indicator of environmental stress in cnidarian zooxanthellae.