The Justice Department, which had charged Bessemer, Ala., with a "racially selective annexation policy," reached an agreement yesterday that requires the town to change its form of government.

The agreement came in a consent decree in which the Birmingham suburb agreed to switch from an at-large commission to a mayor and council with single-member districts. A spokesman for Assistant Attorney General William Bradford Reynolds said it was the first annexation case in which the department has used new voting rights authority.

The department charged last year that Bessemer diluted black voting strength by annexing neighboring areas after blacks became politically active in 1950.

The annexations reduced the town's population from 61 to 51 percent black by 1980, although the population within the original 1950 boundaries is nearly two-thirds black.

Beginning in 1986, blacks will have "a substantial voting majority" in half of the new council districts, the department said.

In another voting rights case, the Justice Department sought a restraining order to force Texas to obtain advance permission within five days for a special election runoff in its 1st Congressional District.

The filing continued a battle between Reynolds and Texas Gov. Mark White (D). Reynolds sued the governor last week, saying he violated the Voting Rights Act by scheduling a June 29 special election and an Aug. 3 runoff without advance approval from the Justice Department.

White said there is no need to submit the dates for approval because the state's special election procedures have been approved for years. Texas Democrats have accused Justice of a political ploy to lower voter turnout.

But the department, which has stopped short of trying to block the election, said it had offered to approve the dates quickly. The special election was called after Rep. Sam B. Hall (D-Tex.) resigned to become a federal judge.