The San Jose scale, Aspidiotus perniciosus Comstock, is predominantly a bark-inhabiting species and develops most successfully on limbs in the top of the tree. Incrustations of scales often form on the bark, sometimes on the fruit, and never on the leaves. Numbers of 150 per cm2 are common on the bark, but they rarely exceed 10 on the fruit or 13 on the leaves. Mortality rate on apple trees is greatest on the leaves, somewhat less on the bark, and least on the fruit. Females occur most frequently on the bark and males most often on the leaves. The very few females that develop on leaves seldom reproduce.The distribution of the San Jose scale is quite variable throughout a tree but is more uniform on specific limbs and old twigs. Distribution on twigs less than 1 year old is never uniform; the basal half of these young twigs is always more heavily colonized than the distal half. Few males develop on young twigs and mortality of all scales is greater on this bark than on limbs and old twigs. The San Jose scale has a greater and more variable mortality on the young twig mainly because this wood is more readily injured by feeding of the scale and low winter temperatures. Thus, twigs up to 1 year old are good sample units only in light infestations and following mild winters. The older twig is the most reliable sample for biological surveys or for assessing insecticide treatments but it is more difficult to examine because it usually contains dead scales of many generations.