Seven persons have been arrested in Charles and Prince George's counties in recent days in what investigators say was a scheme to use counterfeit $1 and $5 bills in automatic bill changing machines.

At least three businesses in Charles County experienced losses through use of the counterfeit bills -- one losing more than $800 -- and several businesses in Prince George's also were hit, according to local investigators and Joseph R. Coppola, a spokesman for the Secret Service in Baltimore.

After the arrests of six suspects last Saturday at a 24-hour carwash in Waldorf, Md., police arrested James Rembo Jr., 21, of Largo on Wednesday and charged him with felony counterfeiting and fraud. Coppola said Rembo is accused of using a copy machine in his father's office in Clinton to make counterfeit bills. Coppola said a copy machine and $409 in counterfeit bills were seized.

Darcy Erion, manager of the carwash in Waldorf, said a counterfeit $5 bill showed up there June 26, but because it was only one fake bill he didn't call police. "Then the whole thing just ballooned," he said. "On the 27th I got 34 fake fives . . . . They took me for $170 that night."

After that, Erion said he stopped putting many quarters in the bill changing machine, which feeds coins to customers who want to use equipment in the carwash. "But on July 4 I put in an extra handful and they got another $170 that night . . . . There was no real pattern to the operation. They didn't have to break in to get at the machine, but I ended up with a lot of worthless paper in the morning."

The arrests last Saturday came after Charles County sheriff's deputies kept the establishment under surveillance. Three Prince George's County juveniles and James C. Saculles, 18, of Camp Springs, Bret M. Hoyt, 18, of Suitland and Stephen A. Donovan, 19, of the District of Columbia were charged with misdemeanor theft, according to Sgt. Casey McDevitt of the sheriff's department.

After notifying police, Erion also called Hamilton Scale, manufacturer of the bill-changing machine in Toledo, Ohio. "They were very surprised that a plain black-and-white reprint could do that," Erion said.

The problem was traced to a worn-out computer chip in the machine's validator, which, once replaced, eliminated the problem, Erion said.

Investigator Joseph Monminy of the Charles County Sheriff's Department said there have been similar thefts using counterfeit bills at a carwash in Bryans Road, Md., and another case in Waldorf.

Prince George's investigators said they had gotten complaints from several businesses, but declined to identify them. They said they have recovered $200 in fake $5 and $1 bills from various county businesses and that the counterfeiting incidents "may not be confined to carwashes or laundromats."