Postal authorities in Canada are trying to track down a "rogue elf" who has been writing obscene letters to children, supposedly on behalf of Father Christmas.
Thousands of children in Canada address their Christmas wishlists to a special address for Santa Claus, which has a festive H0H0H0 zip code.
About 11,000 postal employees and volunteers reply to the letters on behalf of Santa, sending out about a million letters to children in Canada and around the world every year.
But 10 letters sent to children in the Ottawa area have contained obscene messages.
Two of them were received by Rosalyn Da Costa's children Maya, 2, and Colton, 10.
She told the Ottawa Citizen: "I told Maya: 'There's a letter from Santa just for you, let's read it.' We sat down on the couch, I opened the letter and began to read. My mouth dropped open: 'Oh, my God!'"
Each Santa letter Canada Post delivers contains the same main message with a hand-written personal postscript.
Maya's personal P.S. read: "This letter is too long, you dumb s***."
The P.S. on Colton's letter was even ruder.
A spokeswoman for Canada Post told the paper: "We believe there is just one rogue elf out there."
She added: "Everybody here is so shocked. Disappointed doesn't even begin to describe how we feel."
The Write To Santa program, which has been running for 26 years, has been closed in Ottawa until the offender has been caught.
It is not the first time that elves had added spiteful messages to Santa's reply.
In 1999, an Oshawa seven-year-old received a letter from Canada Post calling him "one greedy little boy."