From April to July of 1956, the field population count of the common cabbage butterfly was carried out to study the factors affecting the natural mortality on a cabbage field of National Institute of Agricultural Sciences. In the present paper the authors described the results of this study and concurrently attempted a comparison with the results of the preceding observations already reported (MIYASHITA, ITÔ & GOTOH, 1956).<br>The cabbage field of 1 acre in area was covered by a 8-mesh net to keep the butterflies off. This field was also divided by the net into two sections both of which contained 128 cabbage plants.<br>Ninety-four second brood butterflies (44 males and 50 females) collected in the suburb of Tokyo were released into section B of the experimental field on April 21, and were eliminated after two days.<br>Counts of numbers of eggs, larvae, pupae, and individuals succumbed to parasite and disease were conducted on all plants until the whole insects pupated. All the pupae were removed from the field and kept in the laboratory. When the third brood butterflies had emerged from these pupae, each one-thirds of butterflies emerging every day were released on the same day into section A of the field, and were allowed to oviposit until their deaths. Fourty pairs of the third brood butterflies were also liberated into section B on June 8 and eliminated on June 10. Counts were made in both sections, as in the 2nd brood.<br>The results of these counts are shown in Tables 1, 2 and 3, and the survivorship curves calculated from these tables in Fig. 1.<br>In every case, remarkable reductions in the number of individuals were found at three developmental stages, i.e. the egg and the first larval instar, the last larval instar, and the pupal stage. The second, third and fourth larval instars showed no conspicuous decrease.<br>From the comparison of six different generations reported here and in the previous paper, the following conclusions could be drawn.<br>1. Except the mid-summer generation of 1954, mortalities of the egg and the first larval instar were estimated as 50 to 70 per cent of the initial number of eggs. These mortalities seemed to be caused by the mechanical effect of weather and by the predation of polyphagous predators. In mid-summer generation of 1954, predation of eggs by Orius bugs played a dominant rôle in an extraordinary high mortality.<br>2. Mortalities of the last larval instar varied in different seasons and were always attributable to the “wilt” disease and to the parasitism by Apanteles glomeratus. The rôles of these factors seemed to be intensified by genaration by generation except that the parasitism decreased temporarily in the mid-summer generation. The increase rates from the preceding generation were larger in the second and the third generations of 1956 than in the third generation of 1955. In 1956, this high increase mainly due to the less propagation of the parasite.<br>3. Mortality at the pupal stage was due chiefly to a pupal parasite, Pteromalus puparum. Parasitism of this species was higher in the third than in the second brood.<br>In conclusion, abundance of the common cabbage butterflies seemed to be controlled by the mechanical effect of weather and the activity of predators, two species of parasites and “wilt” disease. The weather and the predator affect the early developmental stages, while parasites and the disease affect the last larval and pupal stages. Temporary decrease of the population in the mid-summer generation may be attributable to the density-dependent increase in the activities of biotic factors and the harmful physiological effect of high temperatures. As for the activity of predation against the fullgrown larvae and the overwintering pupae, future studies are needed.