ABSTRACTThin‐sectioning and freeze‐etching electron microscopy were applied to explore the structure and the temperature‐ and Ca2+‐response of the different host and parasite membranes during intraerythrocytic development ofPlasmodium knowlesiinMacacca mulatta. The plasma membrane of uninfected erythrocytes is temperature‐ and Ca2+‐responsive: chilling to 4°C and exposure to 5 mM Ca2+induces a slight decrease in IMP‐frequency and the emergence of small IMP‐devoid patches on P‐faces. On parasite infection, the erythrocyte membrane becomes modified as indicated by an enhanced temperature‐response and the appearance of caveolae, ca. 70–90 nm in diameter. The frequency of these caveolae is increased in schizont‐infected erythrocytes. Moreover, electron dense plaques, ca. 40 nm in width, appear just beneath the erythrocyte membrane in late trophozoites and schizonts, thus indicating a further modification of the host cell membrane during parasite development. The membrane of the parasitophorous vacuole, derived from the host plasma membrane, dramatically reduces the IMP‐frequency especially on the P‐face upon parasite infection. This leads to an apparent reversal of the IMP‐distribution persisting throughout the whole infection cycle. The parasite plasma membrane forms local compaction domains with the nuclear envelope in ca. 30% of the ring‐stages and trophozoites, which disappear in late trophozoites and schizonts. Moreover, the IMP‐frequency on plasma membrane fracture faces almost doubles during parasite development. Chilling induces a decrease in the IMP‐frequency on P‐faces of the plasma membrane. Surprisingly, however, the parasite plasma membrane and the vacuolar membrane respond to externally applied Ca2+with almost a doubling of the IMP‐frequency. The different parasite endomembranes also undergo characteri