Abstract: By using the electronic microscopic (TEM) method, studies of the patho-genicity of Bacillus thuringiensis var. galleriac strain matsucoccosae against Malsucoeeus matsumurae were carried out in 1987~1988 in our university. Spray the bacterium suspension (6~10) x 108/ml,to the egg-sac of the coccid 12~72hours before hatch, the bacterium have been found in the digestive tract of the coccid in spore stage (Plate I .1). The microvillus and cell membrane of the mid-gut cells have been dissolved by the toxin secreted by the bacterium, only some of the intra-connections between cells left, Coccids in the control groups sprayed with free-germ water are normal (Plate I: 2), Beginning from 48~72 hours after their hatch, some of the bacterium spores in the digestive tract turned into vegetative form and penetrated into the body coelom of the coccid in the bacterium treated groups (Plate I . 3 ; Plate Ⅱ. 1 ) The secreted toxin dissolved the membranes of the cell nuclei (Plate Ⅱ. 2 ) , But those in control groups are normal (Plate Ⅱ: 3). After that, a series of tissues inside the coccid would be destroyed. The order is as follows: connective tissues, fat body (Platel. 4 ), and normal (Plate Ⅱ, 5), muscles and nerve cells. Since some of the baeterium were found near the anus of the coccid and no partition would appear between the digestive tract and the outside through the anus Of the coccid, therefore, the bacterium may penetrate into the coccid through the anus to the digestive tract in the late stage of embryo. The bacterium may have a dormant period in spore stage in the digestive tract.Spray the bacterium suspensions directly to the crawlers of the coccid, no bacteria has been found inside the body except some of those crawlers in high moisture or immersed in bacterium suspension for some time, the bacterium and their parasporal inclusions have been found in the respiratory system, tracheae, (Plate Ⅱ, 6 ),