More than half the people who have signed up to invest up to 10 percent of their pay in the new civil service tax-deferred investment program are age 35 or younger, according to the Thrift Investment Board, which manages the fund, currently growing at a rate of more than $1 million a day.

As might be expected, most of the interest in the new plan is coming from middle- and upper-income workers in agencies with high grade levels and large percentages of professional and technical employes.

One reason so many relatively young employes are participating in the program is that they are recent hires who can benefit from the most generous investment options of a new pension plan set up for them. Most workers hired before 1984 can't invest as much or get matching contributions from the government.

The investment plan is part of the new Federal Employees Retirement System. It covers most post-1983 hires. Its generous investment options allow workers to invest up to 10 percent of their salaries (or a maximum of $7,000 this year) in the program.

Anyone putting in 5 percent or more gets a 5 percent government match that is also tax-deferred.

All FERS employes have accounts opened for them and get a regular 1 percent contribution from the government each paycheck whether they invest or not.

Because of this freebie feature, nearly half the total accounts opened by or for U.S. workers contain no employe contributions.

The highest proportion of new employes who are putting money into their own investment accounts -- 48 percent -- is at the Environmental Protection Agency. Forty-seven percent of the Energy Department's FERS-covered staff members are contributing money to their own accounts, as are 42 percent of the newest hires in the Transportation Department.

Later this year, employes covered by the old pension plan (generally pre-1984 hires) will have the option of moving to the FERS plan, trading reduced civil service pension benefits for the option to invest more and get matching government contributions to their accounts. Most pre-1984 hires can invest only 5 percent of pay, with no match from the government.

Despite sizable turnouts in some agencies, most U.S. workers -- especially lower-income employes and those outside the Washington area -- have been slow to sign up for the investment plan, even though it is equal to or better than similar plans offered most workers in the private sector.

Since enrollments started in April, 831,107 tax-deferred investment accounts have been opened with about $292 million.

Pre-1984 hires (limited to 5 percent contribution rates) total about 2 million people. As of the first week in June, 274,231 of them had opened accounts, investing from 1 to 5 percent of pay with no government match.

Starting next month, those pre-1984 hires must decide whether to stay in the current pension program or move to the new FERS plan. For many a major consideration will be whether their benefits will be better if they stick with the old plan, which has a more generous civil service pension, or move into the new system, which offers reduced civil service benefits but the chance to invest more and maximize those investments with matching government contributions.