Abstract Mitochondrial mRNAs in the green algal class Chlorophyceae have non‐template 3' oligo‐cytosine‐rich additions, also known as oligo(C), polycitydylation, or poly(C) “tails.” These oligonucleotide additions are believed to be unique to green algal mitochondria, as none have been observed in any other organism. Among algae, oligocitydylation had only been observed in species within the Chlorophyceae, with no evidence of it occurring in other taxonomic classes. In this study, evidence is presented that mitochondrial mRNA oligocitydylation occurs in the genus Ulva , demonstrating its presence in the Class Ulvophyceae and Order Ulvales. Two other species in Ulvophyceae, Bryopsis plumosa and Codium fragile , from the Order Bryopsidales were also screened, but no evidence of oligonucleotide additions was observed. This demonstrates that oligocitydylation occurs in Ulvophyceae but suggests it may be limited to the Ulvales. Three species from Trebouxiophyceae were also screened, and there was no evidence of oligonucleotide additions, suggesting it does not occur in this clade. These results demonstrate that oligocitydylation does occur outside of the Chlorophyceae but only in the closely related Ulvales clade, suggesting this could be a biochemical synapomorphy shared by these groups.