The Southern Poverty Law Center said Thursday that it had fired its co-founder and chief trial lawyer, Morris Dees, after nearly a half-century, during which he helped build the organization into a fearsome powerhouse that focused on hate crimes and with an endowment that approached half a billion dollars.

The group’s president, Richard Cohen, did not give a specific reason for the dismissal of Mr. Dees, 82, on Wednesday. But Mr. Cohen said in a statement that as a civil-rights group, the S.P.L.C. was “committed to ensuring that the conduct of our staff reflects the mission of the organization and the values we hope to instill in the world.”

“When one of our own fails to meet those standards, no matter his or her role in the organization, we take it seriously and must take appropriate action,” Mr. Cohen said.

Mr. Cohen’s statement suggested that Mr. Dees’s firing was linked to workplace conduct. He said the center, which is based in Montgomery, Ala., had requested “a comprehensive assessment of our internal climate and workplace practices” in a bid to ensure that the organization was a place where “all voices are heard and all staff members are respected.”