The diatom Coscinodiscus wailesii GRAN is one of the harmful species that causes serious damage to Nori (Porphyra) culture in the coastal sea of Japan. We found resting cells of C. wailesii in sediments of Harima-Nada, Seto Inland Sea, Japan. The resting cells in the sediments were distinguished from vegetative cells because the cytoplasms of resting cells were partially separated from their frustules and were concentrated in the center of the cell. When isolated resting cells were incubated at 20°C under continuous illumination of 65μmol· m^-2, s^-1, these cells showed a quick response of morphological change into vegetative cells. About 70% of the resting cells rejuvenated and about 80% of the rejuvenated cells finished their first cell divisions within 48 hours. This diatom can survive under dark conditions for long periods (at least 15 months) and can rejuvenate rapidly with adequate light, temperature and nutrients. These results suggest that resting cells might be responsible in seeding for the bloom of this diatom in Harima-Nada.