Large membrane-bound inclusions were clearly visible within the gastrodermis and lipid-containing cells of planulae and settled larvae of the zooxanthellate coral, Pocillopora damicornis after fixation or freeze-substitution. We suggest that these inclusions may be a novel potassium (kalium) chloride concentrating organelle, for which we propose the name kalisome. The inclusions were more abundant in settled larvae than in planulae and were not present in mature polyps. In planulae of the azooxanthellate coral, Dendrophyllia sp. these inclusions were extremely rare. Quantitative X-ray microanalysis of freeze-substituted preparations showed that the inclusions in P. damicornis settled larvae contained very high, positively correlated, concentrations of K (2.5 mol x kg(-1)) and Cl (2.5 mol x kg(-1)). Lower concentrations of both K (1.2 mol x kg(-1)) and Cl (1.3 mol x kg(-1)) were detected in P. damicornis planulae, yet higher concentrations were measured in Dendrophyllia planulae (K=6.0 mol x kg(-1); Cl=5.1 mol x kg(-1)). No significant (P>0.05) differences in concentration were observed between inclusions in freeze-substituted and freeze-dried sections of planulae. Symbiotic algae (zooxanthellae) in P. damicornis planulae and settled larvae also contained deposits with high levels of K and Cl, but these were not positively correlated and no structures associated with them were retained by fixation. Significant (P<0.05) concentration differences were also observed between deposits in freeze-substituted and freeze-dried sections. However, similar to 'kalisomes,' zooxanthellae deposits were more abundant in settled larvae than planulae and absent in mature polyps. Higher concentrations of K and Cl were also detected in settled larvae (K=0.7 mol x kg(-1); Cl=1.1 mol x kg(-1)) in comparison to planulae (K=0.4 mol x kg(-1); Cl=0.5 mol x kg(-1)).