Donna Jackson of Eugene, Ore., works for the nation's largest employer as one of 2 million toilers in the federal government's permanent civilian work force.

Well, almost.

In each of the last five years, she has typed up contractors' pay vouchers at Willamette National Forest on every work day of the year but one.

That day -- the day the government officially counts its workers -- she is taken off the rolls along with unknown thousands of other bureaucrats to enable the federal work force to avoid breaking its White House-set manpower ceilings.

In an era of increasing public concern about the size of the federal government, one way the government keeps growing is by hiring full-time part-timers such as Jackson.

They amount to a virtual shadow work force, whose exact dimensions are unknown, but the obviously increasing numbers are both acknowledged and effectively ignored by those charged with containing governmental growth.

"Whenever we find out about it, we do everything we can to stop it," said one official with the Office of Management and Budget, the arm of the presidency empowered to enforce limits on federal growth.

But he added, the most his small agency can do, is "issue instructions not to do it."

Inquiries to the White House about full-time part-timers are referred to OMB.

Nearly one out of every 20 workers in the civilian federal work force, excluding the Postal Service, holds a temporary, part-time or special assignment job that is not counted when the full-time government payroll is measured.

While many of those are legitimate part-time or seasonal workers, a growing number, OMB officials say, work virtually full-time. Those who deal with such workers refer to them as "25-and-ones," since many are kept on federal payrolls for 25 of the government's 26 two-week pay period each year and removed for one.

In the U.S. Forest Service, according to OMB estimates, some 7,000 to 8,000 workers each year are 25-and-ones, who provide the agency with an effective manpower increase of from 32 to 36 per cent over its official limit of 21,950 full-time employes.

"Machinations" like those at the Forest Service have been going on for years, but have become significant only in the last three, according to one OMB official.

Similar abuses have occurred in the Department of Housing and Urban Development, the National Park Service, the Tennessee Valley Authority and other agencies, the OMB says.

"The real problem is that a lot of agencies have more money than personnel slots, and if they are really ingenious, they'll find some way to use it to get more bodies," said another veteran OMB bureaucrat.

"But I don't feel this is entirely to their discredit," he added. "These are officials dedicated to their programs. They just want to get the job done."

When Donna Jackson was first hired, her employers explained to her that there were no real disadvantages in her off-beat job status, she said, though she has since learned that she is shorted a bit in some fringe benefits like insurance and vacations.

She is on a waiting list to be "converted" into a full-fledged job, but she said she doesn't know when that will happen.

"This puts a real strain on these people, causes morale problems," said Forest Service official Robert Gordon. "But we need them, and it's the only way we can bring them on."

Rep. Herbert Harris, (D-Va.) who represents a large constituency of federal workers, contends that the real problem is the artificially low constraints on the number of federal employes and has urged that they be raised where needed.

The 25-and-one gambit is just one way in which the government expands without adding to its official employe count. Another more common practice is to contract jobs out.

However, the federal contracting rules are so esoteric and complicated that the Forest Service has not yet developed a full complement of in-house contracting experts, Gordon said.

In any case, in certain specialized fields, such as timber stand improvement in remote areas, there is sometimes a shortage of contractors available to do the job, he added.

In the old days in the Forest Service, the 25-and-ones used to remain in limbo indefinitely, according to Janet Cooper, general counsel to the National Federation of Federal Employes (NFFE).

But about two years ago, the union, which represents 13,000 Forest Service employes, convinced the agency at least to hire them based on seniority, as jobs became available, she said.

In another case, hundreds of employes of the Census Bureau were recorded as "temporaries" and denied benefits, even though they worked full time for 10 or even 20 years, according to Cooper. After organized protests, those workers are now gradually being hired as regulars, she said.

The most promising solution to the 25-and-one problem would be to base the federal employment limits on the number of hours worked during the year instead of the number of bodies working at year's end, one OMB official said. That proposed system is in experimental stages in some agencies.

However, the official said, OMB director James McIntyre has delayed use of this plan because of objections from several departments that they would "lose flexibility."

Historically, the public has equated the number of federal employes with the "pervasiveness of big government," the official said.

Since World War II, there has been only a two year period, during the Eisenhower administration, when there were no employment limits, to the best of the expert's recollection.

He couldn't remember how much the federal population grew as a result, but he said, "It was enough to make them put the controls back on."

Later, during the Johnson administration, the controls were taken off the number of temporary or part-time workers, leaving the full-time employes under the limit. During a none-month period, "at least 60,000 or 70,000 employes were added, "the official said, "and Johnson promptly put the controls back on."