Cultured human erythrocytes infected with Plasmodium falciparum were studied by freeze-fracture electron microscopy. Special emphasis was placed upon the formation of the membrane surface excrescences (‘knobs’) found on red cells containing mature parasites. Knobs were visualized as conoid projections of the protoplasmic fracture face (PF) and depressions of the exoplasmic fracture face (EF). Knob formation was correlated with parasite growth and, on the basis of the organization of intramembranous particles (IMP) in the PF leaflet, a series of changes associated with parasite maturation was discerned: (1) a focal IMP cluster with minimal erythrocyte membrane elevation; (2) an elevated central IMP cluster surrounded by an IMP-free zone and concentric IMP ring; (3) maximal erythrocyte membrane deformation, concomitant with a loss of obvious IMP organization. Subtle changes in PF IMP organization were seen with knob formation and parasite maturation, including an apparent lateral partitioning of endogenous red cell membrane proteins between knobby or knob-free membrane areas in trophozoite-infected cells. IMP size distributions of the PF were shifted toward smaller particles in schizont-infected cells. Parasite development did not affect IMP densities in the PF; however, a decrease from 464 +/− 106 micron-2 to 374 +/− 94 micron-2 was seen in the EF of schizont-infected cells. IMP densities were similar over knobs and knob-free areas of either membrane leaflet, and there was no apparent EF IMP reorganization associated with the presence of knobs. These findings indicate that dynamic membrane changes are associated with knob formation and parasite maturation.