Allogromiids are organic-walled or agglutinated, single-chambered Foraminifera, common in deep-sea and polar benthic communities. The simple forms and paucity of distinctive features make allogromiid identification difficult by traditional means. Molecular phylogenetic methods offer alternative tools for species identification and are used here to investigate allogromiid diversity. We obtained 135 partial small-subunit ribosomal DNA sequences of allogromiids collected in Explorers Cove, McMurdo Sound, Antarctica. In contrast to the 27 morphotypes identified, phylogenetic analysis revealed 49 molecular types (considered separate species) that differ by more than 5% of sequence divergence. The 49 genetic types form 28 molecular supra-groups that differ by more than 20% and probably represent distinct genera or families. Large genetic distances separating the molecular types indicate unexpectedly high taxonomic diversity. Comparison of our data with sequences of non-Antarctic allogromiids suggests that Explorers Cove species might be endemic and only distantly related to comparable northern hemisphere fauna.