The primary structure of 5.8S rRNA from Chaetomorpha moniligera, one of green algae, was determined by enzymatic method. It consists of 158 nucleotides and contains a 2`-O-methylated purine nucleoside at the position of 42 and a 2`-O-methylated guanosine at the position of 76. The 5.8S rRNA from Chaetomorpha moniligera has 80% and 78% sequence homology with those of Chlamydomonas reinhardii, another green algae, and of yeast, respectively. We have constructed the possible secondary structure of 5.8S rRNA from Chaetomorpha moniligera and we have found that the constructed secondary structure fits very well for the formation of complex with 26S rRNA from yeast as was proposed by Ursi et al (1983). These findings suggest that the sequences of eukaryotic 5.8S rRNA are well conserved during the evolution.