The effects of salinity on the incidence of pseudo-auxospore formation and the size of initial cells were examined at 15, 20, 25, 30, and 35PSU (Practical Salinity Unit) during vegetative cell enlargement of the giant diatom Coscinodiscus wailesii Gran grown in culture. Mean percentages of pseudo-auxospore formation ranged from 1.6 ± 1.6% (mean ± SD) at 15PSU to 42.4 ± 14.7% at 30PSU under experimental conditions, and minimum and maximum percentages were 0.2% at 15PSU and 66.6% at 30PSU, respectively. The mean percentage at 15PSU was much lower than those at other salinities. The optimal salinity for vegetative cell enlargement was 30PSU. Mean valve diameters of initial cells ranged from 383 ± 21 μm (mean ± SD) at 35PSU to 417 ± 16 μm at 20PSU under the incubation conditions. The minimum and maximum sizes of initial cells were 295 μm at 35PSU and 505 μm at 25PSU, i.e. 1.2–2.1 times larger than the mean size, 240 ± 5 μm of the parent cells. Although a few pseudo-auxospores were formed at 15PSU, their shapes were abnormal and most of diem did not divide vegetatively. The size of initial cells was significantly smaller at higher salinities. In nature, therefore, salinity is presumably an important environmental factor influencing size restoration of this diatom by vegetative cell enlargement.