In the account books of the American Express Co., he was Frank E. Thomas, Frank Edward Thomas, Frank Thomas Jr. twice, and six different Frank Thomases with different addresses across the country.

Yesterday, in federal court here, where he pleaded guilty to defrauding American Express and 30 other credit card companies out of more than $55,000 by using 70 different credit accounts, he identified himself as Frank Edward Thomas Jr., 32, of 9549 Kilimanjaro Rd., Columbia, Md.

The 32-year-old computer systems analyst, who earns approximately $20,000 a year, pleaded guilty to a fraud scheme in which he falsely applied for the credit cards by using variations of his real name and then moved to avoid paying bills he ran up on the cards.

He is the second person in two days to plead guilty to such a credit card scheme here. On Thursday, Richard J. Sheppard, of 4660 Martin Luther King Ave. SE, pleaded guilty to a scheme in which he defrauded 29 credit card companies out of more than $17,000.

Assistant U.S. Attorney William Block of the fraud division said the two prosecutions are the first in this area and have allowed investigators here to refine their investigative techniques to find similar schemes.

"We are well aware these aren't the only two gentlemen out there who are doing this," Block said.

Most persons pay their bills on time because they plan to live in the same area for long periods and because they value their credit rating, Block added.

However, persons who move around frequently like Thomas are able to avoid collection, he added.

Investigators also said the companies for whom Thomas worked would pass on only minimal - if any - information to subsequent employers concerning past credit problems he may have had.

Det. William Prudden of the D.C. Police Department check and fraud squad said the Thomas accounts were brought to the police by a local bank that noticed the similar names in a credit check.

The investigators said some credit card companies in the past have written off such uncollected accounts as losses.

"They are now becoming aware that the problem is so bad it cannot be handled civilly," Block said. "We're trying to tell the public" that numerous federal and local criminal statutes are involved in such a scheme.

Thomas entered his plea specifically to one charge of wire fraud in connection with a telephone loan application he made to Riggs National Bank here.

Thomas could be sentenced to a prison term of up to five years and fined $1,000. U.S. District Court Chief Judge William B. Jones has not yet set a sentencing date.

Sheppard entered his guilty plea to mail fraud charges before U.S. District Judge John Lewis Smith Jr. He also will be sentenced later.