observed at all incubation temperatures. Rhoptry (a third class of apical secretory organelles) secretion by extracellular parasites, as monitored by ROP1, was not detected regardless of the incubation temperature (data not shown). Furthermore, we found that the effect of temperature on parasite invasion was not the result of changes in surface antigen SAG1 expression (Fig. 1). These results suggest that the temperature sensitivity of parasite invasion of host cells is probably caused by an effect on microneme secretion.