Amphistegina are the most common foraminifers with algal endosymbionts found on reefs and carbonate shelves worldwide. Like zooxanthellate corals and other reef organisms with algal symbionts, Amphistegina respond to photoxidative stress by bleaching. This paper documents ultrastructural changes that occur during bleaching under field and laboratory conditions. Nine chambers from the outer whorl of each of 22 normal-appearing and 11 partly bleached specimens of Amphistegina gibbosa, which were collected from Conch Reef, Florida, USA, were examined using transmission electron microscopy. The condition and numbers of algal symbionts, as well as the cell area occupied by 10 other intracellular structures of the host, were quantified. Normal-appearing specimens averaged three times more viable symbionts and less than a fourth as many deteriorating symbionts as partly bleached specimens. Foraminifers experimentally exposed to visible light intensities ≥ 13 μmole photon m⁻² s⁻¹ for 35 d were statistically similar to partly bleached field specimens in the number and condition of symbionts, and in chamber area occupied by the evaluated host structures. Exposure to 32 °C water temperature at 6–8 μmole photon m⁻² s⁻¹ for 28 d induced symbiont loss but did not degrade host endoplasm.