Abstract: Objective To study whether Streptococcus pneumoniae (S.pn) can provoke filamentous actin (F-actin) rearran-gements in vitro through calcium signaling pathways in type Ⅱ pneumocytes(A549 cells), resulting in S.pn invasion of the cells. Method After FITC-phalloidin labeling of F-actin, F-actin rearrangements were observed by S.pn adhesion to type pneumocyte A549 cells. S.pn invasion of A549 cells was determined by pretreating A549 cells with cytochalasin D. To investigate whether F-actin rearrangements could be blocked by Ca²⁺ inhibitors, A549 cells were pretreated with Ca²⁺ inhibitors dantrolene, and loaded in Fura-2/AM probe to determine the concentration of cytosolic free calcium by S.pn adhesion to A549 cells after 30, 60, and 90 min respectively. Results Intact S.pn can promote F-actin rearrangements. Cytochalasin D was able to prevent S.pn invasion of A549 cells. No invasion of A549 cell can be determined at 0.25 μg/ml of cytochalasin D. One subset of the inhibitors of Ca²⁺ signal transduction molecules blocked F-actin rearrangements dose-dependently, and S.pn adhesion of A549 cells for 30, 60, and 90 min increased cytosolic free calcium, reaching (487.5±38.1),(548.2±35.6) and (557.2±47.5) nmol/L, respectively. They were higher than of the control group. Conclusion S.pn can provoke F-actin rearrangements through Ca²⁺ signaling pathways, which further leads to S.pn invasion of A549 cells.