If you're keeping track of deadbeats and folks who don't do right by their customers, put Uncle Sam on the list.

The U.S. Treasury is hiding behind the oldest excuse in the book--"the check is in the mail"--to respond to complaints from citizens who haven't received payments that are due on investments in government Treasury bills.

At the same time, the Treasury is "playing the float" with people's money, telling them it will pay no interest on T-bill checks that don't arrive on time.

The Treasury is telling investors they may have to wait as long as six to eight weeks to get money that is owed them, because the checks have been lost by the Post Office. Or stolen. Or something.

As a result, investors are literally losing interest on investments in Treasury bills, which are supposed to be the safest, soundest investment you can make. Instead of putting money in a bank that could go broke or buying stock in a company that might collapse, many people prefer to invest in America and buy a T-bill.

T-bills are the one of the ways the government borrows to finance the national debt. Like U.S. Savings Bonds, T-bills are sold by the government at a discount from their face value, which is $10,000 for T-bills. When the bill matures--in three months, six months or a year--the buyer is supposed to get the full $10,000 back.

Recently, however, a number of T-bill buyers have been calling The Washington Post, the Treasury and their congressional representatives to complain that redemption checks are not arriving on time.

How widespread the problem might be is impossible to judge. An aide to Maryland Rep. Michael Barnes says nine constituents--mostly in the Kensington area--reported a total of $100,000 in T-bill checks missing this month.

If $100,000 worth of T-bill checks have been stolen in one Montgomery County neighborhood in a single month, that's a major crime that demands investigation by postal inspectors. If $100,000 has been lost in the mails, some postmasters or mail carriers ought to get the sack.

But Tom Creamer, who works with constituents in Mike Barnes' Georgia Avenue office, suspects some of those checks never got out of the Treasury. He says Treasury officials have had trouble coming up with the check numbers, the first piece of information needed to make good on checks that are missing. Creamer also says the delays in replacing checks seem to be getting longer because the Treasury--thanks to budget cuts--is short of people to handle the problem.

What's outrageous about the situation is not that the Treasury Department, which writes hundreds of thousands of T-bill checks, should lose a few. These things happen.

The trouble is with what the Treasury is doing about it. Again, it's not that Treasury employes are shrugging off the complaints.

But the message the Treasury delivers boils down to saying, "That's tough. You'll get your money sooner or later."

Layoffs aren't the cause. It's simply that because of the high interest rates paid on T-bills in the last couple of years, the number of people investing in them has exploded and the staff has not been expanded enough to keep up.

In other words, the Treasury's Bureau of Public Debt has made a management decision that it's better to make people wait for their checks and permit more errors than to hire a few more clerks.

If Treasury officials want to let service to the public deteriorate, that's their prerogative. But that's a judgment no private financial institution would dare to make.

Treasury's position is that its obligation to T-bill buyers is fulfilled once it puts the check in the mail, even if there's no proof the check got to the mail. (Try using that excuse when you owe money to the Internal Revenue Service.)

The obvious way to compensate investors who--through no fault of their own--don't get their T-bill checks on time would be for the Treasury to pay them extra interest. If other financial institutions can pay interest "from the day of deposit to the day of withdrawal," the Treasury ought to pay interest on the funds until the missing check is replaced.

As it is now, the Treasury is playing the float with the citizen's cash, getting as much as two months' free use of the money while the check is reissued.

Treasury officials respond they are not authorized by Congress to pay additional interest in such cases. Have they asked Congress for that authority? "We've had some discussions with congressional staff." Translation: No, they haven't asked.

If it were an ordinary bank rather than the U.S. Treasury that was slow to pay, investors could complain to the Comptroller of the Currency, which happens to be a part of the Treasury Department. A consumer affairs specialist in the comptroller's office said the matter would be referred to bank examiners, but the smartest thing for a customer to do is to complain directly to officials of the institution. "Don't stop with the branch manager," the comptroller's office advises, "Go right to the top."

The acting director of the Bureau of Public Debt is William M. Gregg. The Secretary of Treasury is Donald T. Regan. The U.S. Treasury telephone number is 566-2000.