The reef-dwelling benthic foraminifera, Amphistegina spp., are dependent upon diatom endosymbionts for growth and calcification. Symbiont loss was discovered in A. gibbosa in September 1991 in the Florida Keys when > 80% of post-juvenile (> 0.6 mm in diameter) individuals were mottled or bleached in samples collected from depths of 10-30 m. Suppressed asexual reproduction, congenital deformities, and abnormal shell damage were observed in affected populations.