AS EARLY AS 1959, the Atlantic Refining Company, ARCO's predecessor, signed an antitrust decree following a House investigation. Since then, brushes with the law over allegations of deliberate business practices unfair to consumers and competitors have become part of business as usual for ARCO. Here are recent highlights of the company's record:

1973 ARCO and nine other oil companies signed a $28.7 million settlement of a suit charging them with fixing gasoline prices in three Eastern states from 1955 to 1965.

1973 The Federal Trade Commission charged ARCO and seven other companies with monopolizing the oil business east of the Rockies. A court decision in this massive case is not expected before the late 1980s.

1977 On July 25, a special task force of the Federal Energy Administration reported that ARCO and the 14 other largest refiners might have over-charged American consumers by billions of dollars. Investigations are still ongoing.

1977 A month later, the state of Washington charged ARCO and 9 other firms with conspiring to restrain trade in most areas of the crude and refined oil business.