Comparative studies of oil cells in the air chamber side walls of eight Japanese taxa of Marchantiaceae showed that three morphological groups can be recognized based on this character: (1) Marchantia polymorpha and M. paleacea with conspicuous and whitish oil cells, (2) M. emarginata, M. papillata and M. pinnata with indistinct and nearly hyaline oil cells, and (3) Preissia quadrata without oil cells. This grouping is congruent with the results of existing phylogenetic analyses involving Marchantiaceae taxa. The morphology of oil cells in the side walls of air chambers should be used as a new taxonomic character within the family Marchantiaceae.