Aspects of the in situ ultrastructure of the symbiont of the jellyfish Catostylus sp. are described. The in situ symbiont resembles what has previously been described as the gymnodinioid form of Symbiodinium microadriaticum. The Catostylus sp. symbiont stands apart from other gymnodinioid symbionts in having lipid droplets within its chloroplast and evidence of transfer of lipid across the chloroplast membrane. The ultrastructure features described support the suggestion that lipid droplets are synthesised in the chloroplast and are transported into the cytoplasm of the symbiont.