1. Helmet-shaped egg capsules of a common Japanese periwinkle, Littorina brevicula, were found during the season from January to April in plankton samples hauled regularly throughout two years, 1965∿1966, at a fixed point in Moroiso-Aburatsubo inlet, near Misaki Marine Biological Station. 2. The capsules contained one or two eggs. Those containing one egg were 320∿437μ in diameter (average 375μ), while those containing two eggs were 439∿514μ in diameter (average 477μ). 3. Simultaneous spawning activity during some relatively short period of the day were suggested by the facts that there were considerable quantities of capsules at the same and early developmental stages in some samples, although a female releases a small number of capsules (several dozens) at a single spawning. 4. An abnormal capsule was briefly described.