A merozoite stage parasite/early ring stage parasite (A, parasite on the right side labeled with an asterisk) in an erythrocyte double infected by P. falciparum. Electron micrographs of the fourth (A) and eleventh section (B) of this series are shown as overviews, and the area containing the merozoite stage parasite/early ring stage parasite is shown in Sects. 1 to 16. The asterisk in (A) marks membrane whorls in the merozoite stage parasite/early ring stage parasite. Note that the larger of the two ring-like structures is in contact with the parasite. Higher magnifications of the two ring structures reveal that they are limited on either side by a single unit membrane (P, inset). Pa, parasite; Bars, 1 µm (in B for A and B; in R for C–R), 100 nm (inset in P). Serial ultrathin sectioning of P. falciparum-infected red blood cells, subsequent three-dimensional reconstructions, and in addition examination of Giemsa-stained blood films revealed that intraerythrocytic membrane rings/loops are not isolated structures but are locally in contact with the parasite. They consist either of the parasitophorous vacuolar membrane alone or contain the parasitophorous vacuolar membrane including the plasma membrane of the parasite and small amounts of parasite cytoplasm. We demonstrate that membrane rings/loops represent surface extensions of the parasite that maybe involved in ring stage parasite formation and Maurer's cleft generation at least in a subset of infected red blood cells. Wickert H, Krohne G. Giant membrane rings/loops in the cytosol of P. falciparum-infected erythrocytes and their relation to the parasite. Parasitol Res. 2024 123(9):311