Studies that carefully monitor and attempt to explain causes of massive oyster mortalities are urgently needed. The objective of the present study was to assess the effects of seeding dates on the mortality and reproductive cycle of the oyster Crassostrea gigas. Seeds of two sizes (3.5–4 mm and 1.5–2 mm) were planted during three seasons: early summer, early autumn, and late autumn. The experimental period was 6 mo. Mortalities of C. gigas at culture phase nursery lantern-nets, intermediate lantern-nets, and final lantern-nets were recorded. The reproductive cycle of the oysters grown from 3.5 to 4 mm seeds planted in each season was closely monitored (bimonthly) during the experimental period. In addition, seawater parameters [temperature (daily) and salinity (weekly)] were monitored throughout the experimental period. Results of this study demonstrated that the planting season affected C. gigas mortality after the nursery phase. Lower cumulative mortality was observed for 3.5–4 mm seeds than for 1.5–2 mm seeds when the seeds were planted in early (MarL: 0.29 ± 0.02) and late (JunL: 0.22 ± 0.10) autumn, after 6 mo of culture. The reproductive cycle of seeds planted in the summer exhibited an early maturation process, and those planted in the autumn exhibited late reproductive development.