Abstract: A signalling device for attachment to a rural type mailbox mounted on 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 flag plate pivotally secured to either the mailbox or a support post which is actuated by the opening of the door of the mailbox to cause the flag plate to fall, due to gravity, from a raised, non-signalling, position to a lowered, signalling, position. After the mail is removed from the mailbox, the flag plate is easily raised to reset the signalling device in its raised, non-signalling, position to be reactuated by the mail carrier when he next delivers the mail.