The Fairfax County government is coming under increasing attack, even within the walls of its own bureaucracy, for what critics say is a waning commitment to the environment.

In the most recent manifestation of discontent, Joh H. Thillmann, the county's chief environmental official, has quit in disillusionment. With his departure, the county is left without a single, high-ranking environmental professional with the clout to command the attention of the county executive or the board of supervisors.

Thillmann, who is joining the Interior Department as a policy analyst, said his staff is being reorganized into insignificance. The new "environmental branch" of the county planning office, he said, will not be headed by an environmental specialist, and is packed with what he called "computer jockeys and nonenvironmental types."

"I love my job," Thillmann said in an interview. "I would not have been looking for a new one without the reorganization."

Top Fairfax officials reacted indignantly to suggestions from Thillmann and others that the county is reneging on previous environmental commitments.

"We're the most environmentally conscious county of any in the commonwealth," said Acting County Executive J. Hamilton Lambert yesterday.

"That is a fallacious charge," said Supervisor Martha V. Pennino (D-Centreville). "There's been no pulling back from our commitment. This charge is just an attempt to stop growth in Fairfax County. People keep coming here. What are you going to do -- stack them on top of each other?"

Thillmann's resignation, together with othr rumblings, comes at a critical juncture and during an election year for the supervisors. The county is going through one of its most rapid surges of development. Much of the current growth pressure is coming in some of the most environmentally sensitive areas -- near the Occoquan Reservoir, the drinking water source for more than 600,000 Northern Virginians, above the Potomac River in picturesque Great Falls and in the Difficult Run watershed, which cuts a broad green swath across the northern part of the county.

In one of the most broad ranging criticisms of county policy, the Environmental Quality Advisory Council, a citizen group that advises the board of supervisors, has issued a nine-point bill of complaints on the state of Fairfax's environment.

Last week, in a special meeting with the supervisors, the council tried to take up the points, but it didn't get to the first one. The supervisors told council members to take up the matter with the county staff and then return for another meeting.

"I was a little bit dismayed we did not get the attention of the board on this matter," Carmin C. Caputo, the new chairman of the council, said in an interview. As to the county's record on protecting the environment, Caputo said, "We feel it could be done better."

Another group advising the board, the Tree Commission, has also decided to speak out on what it sees as environmental deterioration. At a meeting in January, the commission said an "emergency situation" exists in the area surrounding the Occoquan Reservoir and urged a moratorium on development until the county has a plan to control pollution caused by storm water runoff.

Thillman said the pace of growth in the county "is absolutely crazy," noting that recently the supervisors hastily scrapped a proposal designed to slow growth in the Occoquan and Great Falls areas even before scheduled public hearings were held.

Some critics say the supervisors began downgrading environmental priorities a year ago when they foced the director of environmental affairs, Donald R. Bowman, to resign and abolished his office.

Bowman was forced out because he wanted the county to join a regional effort to develop voluntary standards for controlling the impact of development in the Occoquan Basin. At the time, the county planning staff said such a move would lead to the county giving up some of its powers on land use control. Fairfax, subsequently, did join the regional effort -- although there are criticisms by some environmentalists that the regional standards won't be stringent enough to make a difference.

After it was abolished last year, Bowman's office was replaced by an "environmental tasl fprce" representing a cross section of county offices. Though the task force has been in existence for a year, Supervisor Alan H. Magazine (D-Mason) said last week he didn't even know it existed.

Samuel A. Finz, the deputy county executive with overall authority in the area of planning and development, said, "It's said I am not an environmentalist. Well, I'm not a developmentlist either."

As for criticism that the county isn't doing enough to protect the environment, Finz said: "When they (critics) see one tree fall, they say (we) look the other way. The point of the matter is you can't stop progress. You try to work things out on a reasonable basis. It often comes down to negotiating and talking with the developer."

Five years ago, when the supervisors were undertaking a massive new planning effort, they committed themselves, in a highly publicized declaration, to "enhancing the quality of life" in Fairfax. But since that time, in the view of many environmental observers, the pendulum has turned in the other direction. Now the critics say they are trying to push it back in the other direction.