The affinity of merozoites of Sarcocystis singaporensis obtained from the lungs of acutely infected rats to muscle cells and other cell lines grown in vitro was examined. Two distinct types of mature schizonts developed in the lungs 11–13 days p.i. with sporocysts: those containing PAS− merozoites (type 1) which mainly reacted with antibodies prepared against sporozoites, and others containing PAS+ merozoites (type 2) which were antigenically close to bradyzoites. When inoculated onto cell cultures, type 1-merozoites induced schizogonic development in brain capillary endothelial cells of the rat. In contrast, type 2-merozoites invaded L6 myoblasts. In long-term cultures (50 days) of L6 cells, zoites transformed to a 8–15 μm long uninucleate stage which, tentatively, could be unizoite sarcocysts. Although the observed dichotomy in merozoite development is unprecedented in this form, evidence from previous work suggests that these observations are relevant to other Sarcocystis species. The presented cell culture system could be a first step towards successful growth of sarcocysts in vitro.