The mechanism of the process leading to cell-cell fusion induced by enveloped viruses at a mildly acidic pH is as yet unknown. In this report we demonstrate that the fusion events induced by three viruses of different families, namely Semliki Forest (togavirus), vesicular stomatitis (rhabdovirus) and influenza (orthomyxovirus), share common features. In all three systems a sudden drop of the intracellular pH—below the critical eextracellular pH required to trigger “fusion from within” (FFWI)—is observed. This influx of protons is specific and not due to a general leakiness of the plasma membrane, and therefore might be caused by the opening of a proton channel.