Abstract: A self-contained signalling device for attachment to a rural type mailbox mounted along the side of a road for alerting the owner of the mailbox that mail has been deposited therein by the mail carrier. The signalling device includes a brightly colored flag carried by a pivotally mounted flag support assembly which is actuated upon the opening of the door of the mailbox to cause the flag to lower itself from a folded non-signalling position to an extended signalling position. After removing mail from the mail box, the signalling device is easily raised by the owner of the box to reset the signalling device to its folded non-signalling, position to be reactuated by the mail carrier when the next delivers mail to the box.