Benthic foraminifera of the family Soritinae are important members of coral reef communities, contributing to carbonate deposition on coral reefs. These giant protists form photosymbiotic associations with microalgae of the genus Symbiodinium. The extent of flexibility in foraminefera-Symbiodinium partnerships is not well understood. While some studies suggest foraminifera exhibit strong specificity with regard to symbiont choice, recent work illustrated that at least a few taxa are able to host >1 symbiont type. We explored the symbiont diversity of a widely distributed soritid foraminifera (Marginopora vertebralis), sampling 369 individuals from 16 populations distributed across a wide latitudinal gradient (31 to 9°S) in the western Pacific Ocean using the internal transcribed spacer region 2 (ITS2) of rDNA. We discovered that M. verte - bralis forms symbiotic associations with a high diversity of Symbiodinium types, which encompassed 27 unique ITS2 rDNA haplotypes from 4 major Symbiodinium clades. Distance-based redundancy analysis revealed that the observed geographic variation in symbiont community composition was correlated with several sea surface temperature parameters. Symbiont diversity was highest at the inshore Great Barrier Reef, in marginal habitats characterized by high seasonal fluctuations in environmental parameters. In those areas we found evidence of mixed infections, with individual hosts harboring multiple symbiont lineages. These findings suggest a high degree of flexibility in foraminifera-Symbiodinium partnerships and highlight the importance of environmental variables in shaping symbiotic associations. We discuss the results in light of the hypo - thesis that within-population symbiont polymorphism and mixed infections may be a mechanism to cope with temporal environmental fluctuations.