1) Moina Macrocopa STRAUS appears in the carp-culture ponds in May, but disappears during following eleven months, from June to April of the following year remaining dormant in the form of winter eggs kept in the bottom silt.
2) Population growth of Moina in the ponds was investigated, and it is classified into three types. The first is characteristic by the sharp decline of the parthenogenetic reproduction which then shifts to the bisexual phase. In the second type the growth shows a fluctuation taking a wave shape, and in this type considerable population growth is expected. The third differs fundamentally from those above mentioned. In this case two distinct species of Moina come into being showing their growth peaks at different times.
3) Community of Moina even in the neighboring ponds shows different patterns, and there its great fluctuations are met with occasionally.
4) The amount of the winter eggs of Moina was from 6×105 to 5×104/m2 on the bottom of the pond, and their survival rate were estimated as 2-3%.
5) 'Sleeping' of the winter eggs of Moina is explained as that it is a kind of inhibition of development enforced by environmental factors, and is not a delayed germination attributable to their own inherent character.
6) Ammonia is undoubtedly effectively related with the sleeping, but the best method to preserve sleeping eggs for a long time is the use of bottom silt. 7) Two types of winter egg formation exist in nature, but in the carp-culture ponds the type of successive formation of the winter eggs by the same mother is common, and this is the reason why there exists such a great amount of winter eggs in the ponds.
8) Non existence of the pseudosexual eggs was experimentally proved.
9) A sexual reproduction of a parasitic fungus, Lagenidium sp., was proved clearly by the infection experiments of its zoospores to the healthy winter eggs of Moina. This fungus is available to know the activity of the egg development.