Ubiquitin is a 76 amino acid protein with a remarkable degree of evolutionary conservation. Ubiquitin plays an essential role in a large number of eukaryotic cellular processes by targeting proteins for proteasome-mediated degradation. Most ubiquitin genes are found as head-to-tail polymers whose products are posttranslationally processed to ubiquitin monomers. We have characterized polyubuiquitin genes from the photosynthetic amoeboflagellate Chlorarachnion sp. CCMP 621 (also known as Bigelowiella natans) and found that they deviate from the canonical polyubiquitin structure in having an amino acid insertion at the junction between each monomer, suggesting that polyubiquitin processing in this organism is unique among eukaryotes. The gene structure indicates that processing likely cleaves monomers at the amino terminus of the insertion. We examined the phylogenetic distribution of the insertion by sequencing polyubiquitin genes from several other eukaryotic groups and found it to be confined to Cercozoa (including Chlorarachnion, Lotharella, Cercomonas, and Euglypha) and Foraminifera (including Reticulomyxa and Haynesina). This character strongly suggests that Cercozoa and Foraminifera are close relatives and form a new “supergroup” of eukaryotes.