Dr. Aris T. Allen - was elected chairman of the Maryland Republican Party today, apparently becoming the first black to head one of the major political parties' state organizations.

Allen, 66, was chosen by acclamation by the 153 members of the Republican State Central Committee in attendance at the special meeting. He had no opposition for the nonpaying party post.

Allen was elected in a special election called to find a successor to David R. Forward, whose midterm resignation became effective today. Forward, who recently became head of a national trade association, said he resigned because his new employers feared that a Democratically controlled Congress would not be responsive to a lobbyist who was also head of the Maryland GOP.

The new state chairman, an Annapolis physician who served for eight years (1966-74) in the state legislature, described himself as a political "pragmatist" whose views fall somewhere in the middle of the Republican spectrum.

He said that his priorities as state GOP head included party unity, an increase in the number of Republicans registered statewide, election of more Republicans to office, and an increased fund-raising effort.

"The Republican Party is the party of the open door," Allen said, welcoming anyone of any political persuasion who has the "ability and willingness to work."

Allen had said earlier that he wants to bring more minority groups, especially blacks, into the Republican Party. "We really do not have a two-party system in this country" because the Republican Party is so week nationwide and throughout Maryland, Allen said.

Allen was nominated by the Republican National Committeeman Lawrence J. Hogan, and the nomination was seconded by Anne Arundel County Executive Robert Pascal and Melissa Martin, chairman of the Prince George's County delegation, among others.

Republican officeholders attending the afternoon meeting included U.S. Rep. Newton I. Steers Jr. of Montgomery County, state Sen. Howard Denis (R-Montgomery): and David R. Scotton, a Washington Suburban Sanitation Commission member. All of them had expressed support for Allen before the meeting.

According to Laurence E. Hulbert, executive director of the State Republican Central Committee, there are only 464,000 registered Republicans in Maryland out of a total voting population of 1.75 million.

The state legislature currently included 126 Democratic and 15 Republican delegates, 39 Democratic and eight Republican state senators, Hulbert said.

Allen is credited with helping save the life of one of the legislature's more conservative members during Allen's career as a state delegate.

On April 12, 1971, Del. Russell O. Hickman collapsed on the floor of the legislature with an apparent heart attack. Allen, a 1944 graduate of the Howard University medical school, pounded on Hickman's chest until other medical assistance could arrive.

Allen will serve out the unexpired part of Forward's four-year-term, which ends in 1978.