Fairfax County's highest-ranking deputy sheriff known by his assosciates as a critic of the sheriff's department currently under investigation, was fired yesterday by Sheriff James D. Swinson.

In a letter to Chief Deputy Myron L. Greenquist Swinson said he acted "in view of the fact that it has been demonstrated over a period of time that we cannot work as a team. I appreciate your past services but I consider that this action is in the best interest of this department." Swinson could not be reached for further comment.

Greenquist said in an interview yesterday he believes he was fired from the $24,600-a-year job because he often told Swinson about alleged wrongdoing at the jail, which is run by the sheriff's department, critized chief jailer John O. Feehan and "told the truth."

He said he thinks his firing was precipated when Swinson was told of a conversation he had with another deputy on Wednesday.

In that conversation, Greenquist said, he discussed the sheriff's action in immediately suspending a deputy who had been charged that day with stealing an immate's jacket.

Greenquist said that in the conversation he was critical of Swinson's action in a different case when the sheriff suspended chief jailer Feehan for one week after Feehan had been fined $25 upon pleading guilty in court to using a prisoner outside the jail for private gain.

Summarizing the view he expressed in the conversation, Greenquist said, "if a man breaks the law, particularly a law enforcement officer, he should be summarily dismissed."

Greenquist said that he has approached Swinson many times about alleged improprities such as taking prisoners out of the jail without court orders to do outside work and transporting inmates in private vehicles.

There are so many good, honest deputies that can be tarnished by so few," Greenquist said. "That's what eats my guts out,"

Fairfax prosecutor Robert F. Horan Jf. Has been investigating allegations of wrongdoing in the sheriff's department for more than a month.

Horan told The Washington Post that he is investigating allegations that an expensive stone fireplace in the sheriff's house was built by an inmate who was an expert stone-mason and also is checking the circumstances in which Swanson received chain-link fencing that divides his $100,000 Great Falls home from an adjacent wooded area.

Horan also is investigating allegations that sheriff's department employes and at least one lawyer in the county regularly had inmates do outside work for them and that some inmates were allowed to visit their wives or girl friends when they should have been in jail.

Former and current deputies told The Post that favoritism in promotions within the sheriff's department is rampant and that inmates were used to prepare treats for the sheriff from food meant for the inmates.

The deputies also said that inmates have been used to prepare food for the sheriff's annual employes' picnic and that food bought for inmates has been taken from the jail for employes' personal consumption.

Swinson consistently has denied any knowledge of the use of prisoners at his home or of any alleged wrongdoing in the department.

Chief jailer Freehan's guilty plea to the misdemeanor charge last month resulted from testimoney at a Circuit Court hearing by a jail inmate who said he had poured concrete for a tool shed being built at the sheriff's home.

Freehan and his uncle had been hired by Swinson to build the shed on their own time in Swinson's backyard.

Freehan first denied using inmates to help with the project, but two days later admitted in court he had lied.

Greenquist, 47, said he joined the sheriff's department in August 1974 after serving 26 years in Amry intelligence. He was made chief deputy in June 1979.