Gametophytic thalli of Porphyra were cryopreserved at liquid nitrogen (LN) temperature with a cryoprotective solution in cryogenic vials. Two clonal cultures of gametophytic thalli of P. yezoensis (TU-1,2) and a clonal gametophytic thalli culture of P. tenera (TU-3) were successfully obtained. These gametophytic thalli cultures were clonally maintained via monospores in the laboratory. When the thalli were pre-frozen below -30°C without cryoprotectants, the survival rate was low. Dimethylsulfoxide (DMSO) considerably decreased cell damage during pre-freezing to -40°C, but did little to decrease the damage upon rapid cooling to the LN temperature. This problem was overcome by using dextran, polyvinylpyrrolidone (PVP) or Ficoll in combination with DMSO. The subsequent survival rate after immersion in LN exceeded 95%. This technique allows for the long-term and facile maintenance of clonal culture of gametophytic thalli of Porphyra.