CHICAGO (AP) _ John T. Geldermann was elected to a one-year term as chairman of the board of governors of the Chicago Mercantile Exchange, defeating Lawrence Rosenberg, another longtime board member.

Both men served terms as chairmen of the Mercantile Exchange, the nation's second-largest futures market, during the 1970s.

Geldermann was one of five officials elected Tuesday by the exchange's 12 board members, who were themselves elected last week. The exchange did not reveal the vote totals.

Geldermann, 63, has been a member of the exchange since 1961 and has served in numerous roles on the board and exchange committees.

He succeeds John F. Sandner, who was appointed to the newly created post of senior policy adviser for a 1-year term. Sandner has completed three years as chairman, the maximum allowed under the exchange's rules.

The board also reappointed Leo Melamed as chairman of the executive committee and special counsel to the board of governors.

Melamed was instrumental in the development of financial futures products in 1972.

Rosenberg will remain the board's legislative liaison for the coming year.

Geldermann co-founded, along with his father, Godfrey J. Geldermann, the Chicago-based trading firm that is now Geldermann Inc., a wholly owned subsidiary of ConAgra, an Omaha, Neb.,-based conglomerate with interests in food, fertilizer and clothing.

Geldermann was seen as a candidate who would maintain the status quo at the exchange while Rosenberg was said to be a reformer who would try to open up the exchange management beyond a small group of executives who dominate it.

As chairman, Geldermann may oversee the implementation of the revolutionary Globex electronic trading network, a joint project of the Mercantile Exchange and London-based Reuters Holdings PLC that is awaiting regulatory approval.

''Our membership is committed to excellence and innovation,'' Geldermann said in a statement. ''My task as the new chairman will be to work with the CME leadership team and the board of governors to ensure that this steady progress continues.''

The board also elected Steven E. Wollack, first vice chairman; James E. Oliff, second vice chairman; M. Scott Gordon, secretary; and Larry B. Leonard, treasurer.

At the Chicago Board of Trade, incumbent chairman Karsten Mahlmann was unopposed in winning a third consecutive term Tuesday. Incumbent vice chairman Patrick H. Arbor was returned to his position after turning back a challenge by Dale Lorenzen.

Lorenzen, a veteran grain trader, was unexpectedly slated over Arbor by the exchange nominating committee, but Arbor quickly obtained the necessary 40 signatures to run on petition against the nominee.