New York ? Mattel Inc.’s newest singing toys are already rising stars on the Internet.

Sing-a-ma-jigs, plush animals that chatter and harmonize, have been popping up on Youtube.com as retailers add them to store shelves. In one homemade video, a man sings a duet of “When the Saints Go Marching In” with a blue Sing-a-ma-jig. In another, a stuffed toy puppy cuddles with a red Sing-a-ma-jig after a rendition of “Where Oh Where Has My Little Dog Gone.”

Mattel is relying on that word-of-song to promote the toys instead of an advertising campaign, and said in June that it doubled production. Since then the El Segundo, Calif.-based toymaker increased the number of Sing-a-ma-jigs “a little bit more” as sales show momentum, said Gina Sirard, vice president of marketing for Mattel.

“We’ve had to go to a few different vendors in Hong Kong to fill the demand that we currently have,” Sirard said in a telephone interview. “There seem to be signs that would point to the fact that this is going to do very well for the holiday season.”

Mattel, also the maker of the Barbie and Fisher-Price toys, is eyeing a bigger piece of holiday sales, which account for about 40 percent of the $21.5 billion annual U.S. toy market, according to researcher NPD Group Inc. in Port Washington, N.Y.

Sing-a-ma-jigs, which Mattel started selling in February at 25 Toys “R” Us locations for $12.99 each, will be a strong contender for the retailer’s Holiday Hot Toy List to be released later this year, said Bob Friedland, a spokesman for Toys “R” Us.