The population of the marine cladocerans was followed up at the two stations in Bingo-Nada, the Inland Sea of Japan, during the period from November 1966 through October 1967.
By the observations made on Penilia schmackeri RICHARD, one of the five species of cladocerans occurred, the following results were obtained:
1) Penilia first appeared in the middle of June; then a sharp increase in number was observed, reaching the maximum value of about 25,000 individuals per cubic meter in early July. After that, the numbers decreased rapidly to zero towards September.
2) Standard length varied within the range of 0.38 to 0.93 mm. There was a general tendency that groups of two sizes seemed to be seen. The larger group was mainly composed of egg-bearing females.
3) The standard length of females with eggs seemed to become gradually smaller from the time of its appearance to the later period, while the fertility as expressed in quantity of eggs or embryos per brood was highest (7.5) at first, followed by a drastic decrease thereafter.
4) Females with summer eggs took up about 25% of the population in the middle of June just after its first appearance. They attained their maximum (69%) in early July, then gradually decreased in number to about 40%. Sexual individuals, namely females bearing a resting egg and males, occurred already in late June, shortly before the highest population density, and continued to be seen until the end of August when Penilia was going to disappear.