Particle ingestion by Acanthamoeba is rapid. Within 40 s bound particles can be surrounded by pseudopods, brought into the cytoplasm, and released as phagosomes into the cytoplasmic stream. In electron micrographs the phagosome appears as a flasklike invagination of the surface. Separation from the surface occurs by fragmentation of the attenuated "neck+ of the invagination. The separated phagosome membrane has a three- to fourfold greater density of intramembrane particles than the plasma membrane from which it derives. This change is evident within 15 min of ingestion and is detectable while the membrane is still tightly apposed to the particle. There is no direct evidence for the mechanism of this increase; no increase in particle density was seen in the membrane at an early stage in the forming phagosomes still connected to the surface. These morphological observations are consistent with chemical analyses, to be reported in a separate communication, that show that the phagosome membrane has a higher protein to phospholipid ratio and a higher glycosphingolipid content than the plasma membrane. Enlarged phagosomes (presumptive phagolysosomes) show multiple small vesiculations of characteristic morphology. The small vesicles are postulated to be the major route of membrane return to the cell surface.