The small subunit ribosomal RNA genes of foraminiferal protists are the largest and most divergent of any eukaryote. We demonstrate that this foraminiferal sequence alteration represents a substantial modification to the small subunit ribosomal RNA structure, including a large (up to 350 nt) novel helix in a very well-conserved portion of the head domain. This modification dates from the beginning of the foraminiferal radiation and, within modern orders, is partially conserved at the sequence level, suggesting that it is a functional part of the ribosome. The pattern of conservation makes it particularly useful for determining lower-taxon relationships in morphologically ambiguous allogromiid foraminifera.