The density of Pseudaphycus malinus Gahan against a given number of host mealybugs, Pseudococcus comstocki (Kuwana) is one of the important factors to be considered in making the mass culture. By using the six different combinations, i.e. about 25, 50, 100, 200, 400 and 800 adult female parasites against 1000 individuals of the third instar host mealybugs reared on one pumpkin, the author examined the number of progeny produced by a single female parasite, parasitism, mortality, sex ratio, the number of days required for the parasite to develop from the egg to the adult stage and the body size of adult female parasites. The number of parasites liberated showed an apparent influence on the number of progeny and parasitism. But little influence was found on the mortality in the case of higher parasite densities. Although the parasitism at the density of 200 parasite was 65.9 percent, that at 400 and 800 parasites was more than 90 percent. The average number of progeny produced by a single female parasite was the greatest at the density of 200 parasites, and that at the density of 100 parasites and of 400 parasites decreased in this order. The density of 200 parasites, however, had one drawback in that the parasitism was low. Therefore, it seems more practical to liberate 400 female parasites against 1000 host mealybugs.