Abstract Monostroma latissimum (Kuetzing) Wittrock is a monostromatic green alga of commercial importance in Japan. Here we report the serendipitous discovery of asexually reproducing specimens collected from Usa, on the Pacific coast of Kochi Prefecture, south-western Japan. Zoids were found to be biflagellate and negatively phototactic. Germination of settled zoids was observed to follow erect-filamentous ontogeny similar to that of the previously reported sexual strain. Moreover, the newly discovered asexual strain had identical sequences of nuclear encoded ITS (Internal Transcribed Spacer) region to that of the sexual strain . On the basis of this finding, we postulate that the ITS sequences may have been maintained in these conspecific strains despite the evolution in sexuality. Relationships were investigated among M. latissimum and other monostromatic taxa within the class Ulvophyceae using ITS sequences in order to understand relative phylogenetic position of this species.