The morphological changes that occur during intraphagocytic digestion of group A streptococci were studied by electron microscopy The first evidence of degradation of the ingested organism was the appearance of reticular changes in the bacterial endoplasm. This was followed by gradual swelling and dissolution of the bacterial wall, with final degradation of all the constitutuents to electron-dense debris. Accompanying changes in the phagocytic cell were observed; they consisted of vacuole formation, fusion of lysosomes with the wall of the yacuole, release of the lysosomal contents into the vacuole, and aggregation of the lysosomal contents around the ingested organism. Changes in the morphology of the organism similar to those observed during intraphagocytic digestion were also obtained by subjecting streptococcal cells to the action of the phage-associated lysin.