Shares of Napster Inc. and other online music providers plummeted Wednesday, as Yahoo Inc. launched a new music subscription service, taking aim at leaders in the booming world of digital music downloads.

Yahoo’s service, which underprices competitors such as L.A.-based Napster Inc., debuts today for as low as $4.99 a month with a one-year subscription. The service, Yahoo Music Unlimited, boasts a catalog of 1 million songs, along with a menu of add-on services, including the ability to build personalized music libraries.

Napster settled down 26.8 percent to $4.65, trading as low as $4.20. Its Napster-To-Go service is $15 per month. Yahoo’s move to get a piece of the online music market dominated by Apple Computer Inc. also weighed down its new rival’s shares, sending Apple slumping 2.2 percent to $35.61 after being down nearly 5 percent for most of the afternoon. RealNetworks Inc., which also has a music service, slid 21.1 percent to $5.76.

Sunnyvale-based Yahoo, the most-visited Internet site, entered the subscription music download business last year with the purchase of MusicMatch Inc. for $160 million. The company’s Yahoo! Music unit is run from Santa Monica.