Former Fairfax City treasurer Frances L. Cox used a "very unsophisticated, very simple embezzlement scheme" to funnel more than $100,000 in cash from the city treasury into her personal bank account, prosecutors charged yesterday.

Deputy Fairfax Commonwealth's Attorney Steven Merril said during opening arguments in Fairfax Circuit Court that Cox had successfully covered the embezzlement scheme and the funds were discovered missing only after she was defeated for reelection last fall. Cox, 56, served as the city's treasurer for 27 years.

"You'd be amazed at how simple it the scheme was," said Merril. "It required only one thing -- that she stay in office."

Cox has pleaded innocent to the indictment charging her with embezzling an unspecified amount of money over a 30-month period. Her lawyer argued yesterday that Cox may have been incompetent as a treasurer, but that she was not involved in any criminal wrongdoing.

"The records of the City of Fairfax were absolutely chaotic," said John H. Rust Jr., Cox's attorney and a Fairfax legislator. "There were many mistakes and errors on the part of Frances and her employes . . . she Cox was aware of the competency problems."

The Fairfax commonwealth's attorney, the city sheriff and other city officials spent almost six months investigating her office after Cox's successor, Ray Birch, resigned after three weeks, saying he had found cash stuffed in drawers, checks left unrecorded and a $156,000 overdraft in the city's bank account.

Merril told the jury hearing the case that investigators discovered Cox had deposited $100,000 in cash into her personal bank account during 1981. Merril added that Cox may have embezzled more than the $100,000 from the city, but that records were in such disarray that investigators tackled only the final 30 months of her tenure.

"The more chaotic the records," Merril said, "the easier it is to conceal the theft."

When Cox made deposits to the city's bank accounts, she frequently substituted much of the cash her office had taken in with checks the office had received by mail for other services, Merril said.

"There are no internal controls over many of these items," Merril said. As a result, he said, some people who were listed as delinquent in paying their taxes in the treasurer's office actually had paid their bills and had the checks to prove it.

Cox falsified deposit statements at least 215 times during the 30-month period covered by the investigation, Merril said. Although other personnel in the treasurer's office occasionally made deposits, "in every case it was Cox that made up false deposit slips," Merril told the jury.

Real estate and personal property tax payments were used to cover about 30 percent of the missing cash, Merril said. Most of the embezzlements were siphoned from over-the-counter payments of bills brought into the office on Saturdays, Merril said.

He said Saturday payments were supposed to be combined with Monday payments and deposited in the city accounts. "The vast majority of the time Saturday's work was completely gone," Merril said.

In today's hearing before Judge Barbara Keenan, the prosecution is expected to present its principal witnesses.