Nine specimens of Pyrenula japonica Kurokawa were collected from various areas in Japan. Photobionts were isolated from their thalli and studied taxonomically. As a result, only one species of green alga,Trentepohlia lagenifera(Hild.) Wille was identified as the photobiont of Pyrenula japonica. This is the first record of this alga from the lichen genus Pyrenula.