WASHINGTON — The 42 million Americans on Medicare will face a dizzying array of choices for government-

subsidized prescription-drug coverage next month when private health-insurance companies begin jockeying to sign them up for the new benefit.

Ten large insurers have been given the green light to market the new drug plans to consumers starting Oct. 1, Medicare officials said yesterday. Because other companies will offer coverage on a regional basis, participants in every state will have at least 11 providers to choose from. In some they will have up to 20 choices, with some providers offering multiple plans.

Enrollment begins Nov. 15. Coverage takes effect Jan. 1.

"Medicare is taking a historic step," said Mark McClellan, administrator for the Centers for Medicare and Medicaid Services. "Thanks to the strong competitive response, everyone with Medicare will be able to choose a drug plan that addresses their individual concerns about cost and coverage and convenience."

No details were available yesterday about specific plans. McClellan said plans with premiums of less than $20 a month will be available in every state. Plans with premiums exceeding $37 should be rare, he said.

Beneficiaries also can choose to get drug coverage through Medicare Advantage plans, which offer comprehensive health care through HMO- and PPO-like systems and serve about 10 percent of Medicare beneficiaries.

The new drug benefit is expected to cost $720 billion over the first 10 years.

The benefit is being rolled out at a time of budget deficits and mounting expenditures for military operations in Iraq and Afghanistan and relief and recovery efforts from hurricanes Katrina and Rita. The pressures are so daunting that some conservative Republican lawmakers have called for delaying implementation of the benefit, but the Bush administration has rejected that idea.

In general, the drug benefit will work like this: After an individual pays a $250 annual deductible, Medicare will cover 75 percent of drug costs up to $2,250. The coverage then stops until the recipient has spent an additional $2,850 out of pocket, after which Medicare covers 95 percent of drug costs.

President Bush has said the new benefit will save retirees an average of $1,300 a year. The government expects up to 30 million people will sign up for it for 2006.

Tricia Neuman, director of the Medicare Policy Project at the nonprofit Kaiser Family Foundation, said the large number of providers means the new benefit is off to a good start. But there is uncertainty over how many people will enroll, how easily they will sort through the options and whether some providers will drop out over time.

Robert Hayes, president of the Medicare Rights Center, a nonprofit group that helps senior citizens navigate Medicare rules and benefits, said the number and complexity of the plans mean consumers will be hard-pressed to make informed decisions. And because the companies can make their pitches through telemarketing, there are concerns about fraud and identity theft, Hayes said.

"It's going to be open season for bad guys to exploit vulnerable older Americans," he said.

The Medicare agency will provide tools to help consumers find a plan that best fits their needs, he said. Beginning in mid-October, consumers will be able to call an 800 number and check the agency's Web site (www.medicare.gov) to get help with plans. Also, next month, the "Medicare & You" handbook distributed to beneficiaries each year will contain details about plan choices.