Background/Aims: Phagocytosis depends on the formation of engulfment pseudopodia surrounding the target. We tested in microglia, monocyte-derived cells in the brain, whether a swelling-activated Cl--current (ICl,swell), required for global cell volume (CV) regulation, also contributes to local expansion and retraction of engulfment pseudopodia. Methods: We used scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) to visualize and quantify the uptake of polystyrene microbeads (MBs) by microglial cells. Flow cytometry was used for cell volume measurments and ICl,swell was measured by whole-cell patch clamp. Results: We found that exposure of microglial BV-2 cells to MBs in Cl--free extracellular solution attenuated MB uptake and that the Cl--channel blockers DIOA, flufenamic acid, NPPB and DCPIB suppressed the uptake of MBs in BV-2 cells and in primary microglial cells. Microglial cells exposed to MBs in the presence of Cl- channel blockers failed to extend engulfment pseudopodia. We observed that cells containing at least three MBs revealed an about twofold increase in current density of ICl, swell compared to cells without MB. Osmotic challenges to stimulate global CV regulation before exposure to MBs modulated phagocytosis. Pre-conditioning of cells in hypo- or hypertonic medium for 12-16 hours caused a decrease in MB uptake. Conclusion: These findings indicate that ICl,swell contributes to formation of engulfment pseudopodia and participates in engulfment and particle uptake in microglial cells.