Transmission electron microscopy was used to determine the intracellular localization of Yersinia pestis growing within cultured resident peritoneal macrophages of mice. Monolayers fixed immediately, or as long as 4.5 h, after infection contained yersiniae closely surrounded by a membrane. The vesicles containing the yersiniae were determined to be phagolysosomes by labeling secondary lysosomes of macrophages with electron-dense particles of thorium dioxide. The presence of the label within vesicles containing yersiniae indicates that this pathogen grows within the lysosomal compartment of its target host cell.