A Baltimore paving company whose president pleaded guilty to conspiracy three weeks ago in connection with a Virginia bid-rigging scheme was charged yesterday in Baltimore federal court with violating federal antitrust laws.

In a corporate plea bargain reported by The Washington Post on Saturday and related to the latest charge, the firm has agreed to pay a $350,000 fine for violating the Sherman Act in securing a construction contract for Maryland Rte. 197. The maximum penalty for the crime is a $1 million fine.

On June 4, Barton S. Mitchell, president of the Baltimore firm, E. Stewart Mitchell, Inc., and of another paving company in Fairfax County, pleaded guilty to conspiracy in U.S. court in Alexandria. That charge stemmed from alleged irregularities involving Northern Virginia paving contracts.

Both the Virginia and Maryland charges are part of a wide-ranging federal investigation into highway bid-rigging that began Dec. 14, 1979, in Tennessee. The probe now encompasses 18 states. Charges have been brought against 152 corporations and 179 executives in 12 states.

Mitchell currently is serving a 150-day sentence at the U.S. correctional institution at Allenwood, Pa. His plea agreement includes a promise to answer all questions put to him by federal grand jurors in the continuing bid-rigging probe.

Mitchell's Maryland firm had been the lowest bidder, at $2.68 million, for the Rte. 197 paving contract. The award was made after the original contractor was declared in default by the state highway administration.