Collections of egg masses of the pine moth, Dendrolimus spectabilis Butler were made at three plots in the Japanese black pine forest on the coast in the vicinity of Fukuoka City during 1960-63. Frequency distribution in the number of eggs per egg mass showed that there were two remarkable size differences of egg masses regardless of plot, year, or generation. These two types of egg masses, viz, small-type and large type, were distinguished from each other by the following size for convenience, though there was observed an overlap in size. Small-type egg masses consist of less than 20 eggs, usually 3 to 6 eggs on an average. Large-type egg masses comprise 21 to 600 or more eggs, and 100 to 200 or more eggs on an average. In general, small-type egg masses were more numerous than large-type egg masses in each generation. D. spectabilis is a pro-ovigenic species, and under laboratory conditions the gravid female lays the greatest number of her eggs within one or two days after emergence. On the other hand, the field census data indicated that the ratio of the small-type egg masses to the large-type ones increased in the later period of oviposition. These facts suggest that the large-type egg masses are laid by the young moth, and that the small-type egg masses are deposited by the older moth.