JUNEAU, Alaska (AP) _ Alaska's unique $250-a-month ''longevity bonus'' for the elderly will be phased out under a measure signed into law Tuesday.

The program was started in 1973 to reward Alaska's pioneers, including miners, fisherman and trappers who had no retirement income.

In 1984, an Alaska Supreme Court ruling struck down a 25-year state residency requirement, and the money has been provided since to thousands of Alaskans age 65 and older who have lived in the state for at least a year.

''The growing number of participants in the program has simply made it unaffordable over the long term,'' said Gov. Walter J. Hickel, who signed the legislation into law Tuesday.

The measure preserves the $250 payments for those enrolled in the program by Dec. 31, reduces it to $200 for those enrolling in 1994, $150 for those signing up in 1995, and $100 in 1996.

Bonuses will be closed to applicants beginning in 1997.

In 1973, enrollment was about 3,600. Enrollment this year was 23,000. It is expected to cost the state $70 million in the coming fiscal year.

The bonus is one of several perks that have attracted older people to Alaska, despite its high cost of living. Others include bargain-rate nursing homes, a generous property-tax exemption and free hunting and fishing licenses.

Some lawyers have questioned whether retaining the bonus for some elderly Alaskans and denying it to others violates the U.S. Constitution's guarantee of equal protection under the law.