1. It is only under the most favourable conditions that oysters (Ostrea edulis) mature in the first sexual phase in the same season as that in which they attach themselves. During this initial phase the oyster functions as a male. 2. Normally the first male phase is experienced on British beds in the summer following that in which the oyster attaches itself and is rapidly followed by the first female phase, the oocytes developing on the walls of the follicles while spermatogenesis proceeds in the lumina. 3. In favourable seasons about a third of a population of one-year-old oysters will spawn as females. In unfavourable years only a few of the heaviest oysters spawn. Following female spawning the second male phase is rapidly assumed. 4. The second male phase is followed by the second female phase, the oocytes developing while spermatogenesis is in progress. Emission of sperm may be continued up to within a few days of egg-spawning. If a male phase is reached towards the close of the season it is followed by a resting phase, without spermatogenesis or large oocytes in the gonad, which persists throughout the winter. Similar resting phases may occur during the summer if environmental conditions are unfavourable. 5. Every two-year-old oyster functions as a female in a favourable season, and thereafter under normal conditions passes through at least two sex-phases each season, functioning both as a male and as a female. Evidence is adduced to show that in populations of adult oysters on British beds 100 per cent, female functioning occurs in favourable seasons. 6. Gonad development is arrested from approximately the end of October until the beginning of April while the water temperature is below about 10°C. Oysters may winter in almost any sex-phase, but spermatogenesis does not occur, although ripe sperm morulae may be carried over from one season to the next. Egg development is arrested during the winter months. 7. Oysters which winter in a transitional phase between a male and a female phase may mature as pure females early in the following summer without any recrudescence of spermatogenesis. There is at this time, therefore, a high proportion of ripe females in the population. 8. Oysters which winter in a dormant male condition mature as good males early in the following season and spawn as females later in the summer. Female spawners may therefore be divided roughly into two groups--early and late spawners--and individual oysters tend to remain in the same group in successive years. 9. Breakdown and absorption of large eggs by phagocytes has been found to occur occasionally during the winter, but the circumstances in which this takes place are not known.